Prepare for Element Call integration (#9224)

* Improve accessibility and testability of Tooltip

Adding a role to Tooltip was motivated by React Testing Library's
reliance on accessibility-related attributes to locate elements.

* Make the ReadyWatchingStore constructor safer

The ReadyWatchingStore constructor previously had a chance to
immediately call onReady, which was dangerous because it was potentially
calling the derived class's onReady at a point when the derived class
hadn't even finished construction yet. In normal usage, I guess this
never was a problem, but it was causing some of the tests I was writing
to crash. This is solved by separating out the onReady call into a start
method.

* Rename 1:1 call components to 'LegacyCall'

to reflect the fact that they're slated for removal, and to not clash
with the new Call code.

* Refactor VideoChannelStore into Call and CallStore

Call is an abstract class that currently only has a Jitsi
implementation, but this will make it easy to later add an Element Call
implementation.

* Remove WidgetReady, ClientReady, and ForceHangupCall hacks

These are no longer used by the new Jitsi call implementation, and can
be removed.

* yarn i18n

* Delete call map entries instead of inserting nulls

* Allow multiple active calls and consolidate call listeners

* Fix a race condition when creating a video room

* Un-hardcode the media device fallback labels

* Apply misc code review fixes

* yarn i18n

* Disconnect from calls more politely on logout

* Fix some strict mode errors

* Fix another updateRoom race condition

src/stores/ModalWidgetStore.ts

@@ -30,10 +30,14 @@ interface IState {
 }
 
 export class ModalWidgetStore extends AsyncStoreWithClient<IState> {
-    private static internalInstance = new ModalWidgetStore();
-    private modalInstance: IHandle<void[]> = null;
-    private openSourceWidgetId: string = null;
-    private openSourceWidgetRoomId: string = null;
+    private static readonly internalInstance = (() => {
+        const instance = new ModalWidgetStore();
+        instance.start();
+        return instance;
+    })();
+    private modalInstance: IHandle<void[]> | null = null;
+    private openSourceWidgetId: string | null = null;
+    private openSourceWidgetRoomId: string | null = null;
 
     private constructor() {
         super(defaultDispatcher, {});