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/components/views/dialogs/UserSettingsDialog.tsx

@@ -159,7 +159,7 @@ export default class UserSettingsDialog extends React.Component<IProps, IState>
             "UserSettingsSecurityPrivacy",
         ));
         // Show the Labs tab if enabled or if there are any active betas
-        if (SdkConfig.get()['showLabsSettings']
+        if (SdkConfig.get("show_labs_settings")
             || SettingsStore.getFeatureSettingNames().some(k => SettingsStore.getBetaInfo(k))
         ) {
             tabs.push(new Tab(