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/structures/auth/Login.tsx

@@ -245,7 +245,7 @@ export default class LoginComponent extends React.PureComponent<IProps, IState>
             } else if (error.httpStatus === 401 || error.httpStatus === 403) {
                 if (error.errcode === 'M_USER_DEACTIVATED') {
                     errorText = _t('This account has been deactivated.');
-                } else if (SdkConfig.get()['disable_custom_urls']) {
+                } else if (SdkConfig.get("disable_custom_urls")) {
                     errorText = (
                         <div>
                             <div>{ _t('Incorrect username and/or password.') }</div>