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

test/components/views/elements/AppTile-test.tsx

@@ -93,7 +93,7 @@ describe("AppTile", () => {
             url: "https://example.com",
             name: "Example 1",
             creatorUserId: cli.getUserId(),
-            avatar_url: null,
+            avatar_url: undefined,
         };
         app2 = {
             id: "1",
@@ -103,7 +103,7 @@ describe("AppTile", () => {
             url: "https://example.com",
             name: "Example 2",
             creatorUserId: cli.getUserId(),
-            avatar_url: null,
+            avatar_url: undefined,
         };
         jest.spyOn(WidgetStore.instance, "getApps").mockImplementation(roomId => {
             if (roomId === "r1") return [app1];