Use & enforce snake_case naming convention on config.json settings (#8062)

* Document and support the established naming convention for config opts

This change:
* Rename `ConfigOptions` to `IConfigOptions` to match code convention/style, plus move it to a dedicated file
* Update comments and surrounding documentation
* Define every single documented option (from element-web's config.md)
* Enable a linter to enforce the convention
* Invent a translation layer for a different change to use
* No attempt to fix build errors from doing this (at this stage)

* Add demo of lint rule in action

* Fix all obvious instances of SdkConfig case conflicts

* Fix tests to use SdkConfig directly

* Add docs to make unset() calling safer

* Appease the linter

* Update documentation to match snake_case_config

* Fix more instances of square brackets off SdkConfig

src/IConfigOptions.ts

@@ -0,0 +1,186 @@
+/*
+Copyright 2016 OpenMarket Ltd
+Copyright 2019 - 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { IClientWellKnown } from "matrix-js-sdk/src/matrix";
+
+import { ValidatedServerConfig } from "./utils/AutoDiscoveryUtils";
+
+// Convention decision: All config options are lower_snake_case
+// We use an isolated file for the interface so we can mess around with the eslint options.
+
+/* eslint-disable camelcase */
+/* eslint @typescript-eslint/naming-convention: ["error", { "selector": "property", "format": ["snake_case"] } ] */
+
+// see element-web config.md for non-developer docs
+export interface IConfigOptions {
+    // dev note: while true that this is arbitrary JSON, it's valuable to enforce that all
+    // config options are documented for "find all usages" sort of searching.
+    // [key: string]: any;
+
+    // Properties of this interface are roughly grouped by their subject matter, such as
+    // "instance customisation", "login stuff", "branding", etc. Use blank lines to denote
+    // a logical separation of properties, but keep similar ones near each other.
+
+    // Exactly one of the following must be supplied
+    default_server_config?: IClientWellKnown; // copy/paste of client well-known
+    default_server_name?: string; // domain to do well-known lookup on
+    default_hs_url?: string; // http url
+
+    default_is_url?: string; // used in combination with default_hs_url, but for the identity server
+
+    // This is intended to be overridden by app startup and not specified by the user
+    // This is also why it's allowed to have an interface that isn't snake_case
+    validated_server_config?: ValidatedServerConfig;
+
+    fallback_hs_url?: string;
+
+    disable_custom_urls?: boolean;
+    disable_guests?: boolean;
+    disable_login_language_selector?: boolean;
+    disable_3pid_login?: boolean;
+
+    brand: string;
+    branding?: {
+        welcome_background_url?: string | string[]; // chosen at random if array
+        auth_header_logo_url?: string;
+        auth_footer_links?: {text: string, url: string}[];
+    };
+
+    map_style_url?: string; // for location-shared maps
+
+    embedded_pages?: {
+        welcome_url?: string;
+        home_url?: string;
+        login_for_welcome?: boolean;
+    };
+
+    permalink_prefix?: string;
+
+    update_base_url?: string;
+    desktop_builds?: {
+        available: boolean;
+        logo: string; // url
+        url: string; // download url
+    };
+    mobile_builds?: {
+        ios?: string; // download url
+        android?: string; // download url
+        fdroid?: string; // download url
+    };
+
+    mobile_guide_toast?: boolean;
+
+    default_theme?: "light" | "dark" | string; // custom themes are strings
+    default_country_code?: string; // ISO 3166 alpha2 country code
+    default_federate?: boolean;
+    default_device_display_name?: string; // for device naming on login+registration
+
+    setting_defaults?: Record<string, any>; // <SettingName, Value>
+
+    integrations_ui_url?: string;
+    integrations_rest_url?: string;
+    integrations_widgets_urls?: string[];
+
+    show_labs_settings?: boolean;
+    features?: Record<string, boolean>; // <FeatureName, EnabledBool>
+
+    bug_report_endpoint_url?: string; // omission disables bug reporting
+    uisi_autorageshake_app?: string;
+    sentry?: {
+        dsn: string;
+        environment?: string; // "production", etc
+    };
+
+    widget_build_url?: string; // url called to replace jitsi/call widget creation
+    audio_stream_url?: string;
+    jitsi?: {
+        preferred_domain: string;
+    };
+    jitsi_widget?: {
+        skip_built_in_welcome_screen?: boolean;
+    };
+    voip?: {
+        obey_asserted_identity?: boolean; // MSC3086
+    };
+
+    logout_redirect_url?: string;
+
+    // sso_immediate_redirect is deprecated in favour of sso_redirect_options.immediate
+    sso_immediate_redirect?: boolean;
+    sso_redirect_options?: ISsoRedirectOptions;
+
+    custom_translations_url?: string;
+
+    report_event?: {
+        admin_message_md: string; // message for how to contact the server owner when reporting an event
+    };
+
+    welcome_user_id?: string;
+
+    room_directory?: {
+        servers: string[];
+    };
+
+    // piwik (matomo) is deprecated in favour of posthog
+    piwik?: false | {
+        url: string; // piwik instance
+        site_id: string;
+        policy_url: string; // cookie policy
+        whitelisted_hs_urls: string[];
+    };
+    posthog?: {
+        project_api_key: string;
+        api_host: string; // hostname
+    };
+    analytics_owner?: string; // defaults to `brand`
+
+    // Server hosting upsell options
+    hosting_signup_link?: string; // slightly different from `host_signup`
+    host_signup?: {
+        brand?: string; // acts as the enabled flag too (truthy == show)
+
+        // Required-ness denotes when `brand` is truthy
+        cookie_policy_url: string;
+        privacy_policy_url: string;
+        terms_of_service_url: string;
+        url: string;
+        domains?: string[];
+    };
+
+    enable_presence_by_hs_url?: Record<string, boolean>; // <HomeserverName, Enabled>
+
+    terms_and_conditions_links?: { url: string, text: string }[];
+
+    latex_maths_delims?: {
+        inline?: {
+            left?: string;
+            right?: string;
+        };
+        display?: {
+            left?: string;
+            right?: string;
+        };
+    };
+
+    sync_timeline_limit?: number;
+    dangerously_allow_unsafe_and_insecure_passwords?: boolean; // developer option
+}
+
+export interface ISsoRedirectOptions {
+    immediate?: boolean;
+    on_welcome_page?: boolean;
+}