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/LoggedInView.tsx

@@ -75,6 +75,7 @@ import RightPanelStore from '../../stores/right-panel/RightPanelStore';
 import { TimelineRenderingType } from "../../contexts/RoomContext";
 import { KeyBindingAction } from "../../accessibility/KeyboardShortcuts";
 import { SwitchSpacePayload } from "../../dispatcher/payloads/SwitchSpacePayload";
+import { IConfigOptions } from "../../IConfigOptions";
 import LeftPanelLiveShareWarning from '../views/beacon/LeftPanelLiveShareWarning';
 
 // We need to fetch each pinned message individually (if we don't already have it)
@@ -103,12 +104,7 @@ interface IProps {
     roomOobData?: IOOBData;
     currentRoomId: string;
     collapseLhs: boolean;
-    config: {
-        piwik: {
-            policyUrl: string;
-        };
-        [key: string]: any;
-    };
+    config: IConfigOptions;
     currentUserId?: string;
     currentGroupId?: string;
     currentGroupIsNew?: boolean;