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/ServerPickerDialog.tsx

@@ -50,7 +50,7 @@ export default class ServerPickerDialog extends React.PureComponent<IProps, ISta
         super(props);
 
         const config = SdkConfig.get();
-        this.defaultServer = config["validated_server_config"] as ValidatedServerConfig;
+        this.defaultServer = config["validated_server_config"];
         const { serverConfig } = this.props;
 
         let otherHomeserver = "";