Add labs flag for Threads Activity Centre (#12137)

* Add `Thread Activity centre` labs flag

* Rename translation string

* Update supportedLevels

* Fix labs subsection test

* Update Threads Activity Centre label

* Make threads activity centre labs flag split out unread counts

Just shows notif & unread counts for main thread if the TAC is enabled.

* Fix tests

* Simpler fix

* Pass in & cache the status of the TAC labs flag

* Pass includeThreads as setting to doesRoomHaveUnreadMessages too

* Fix tests

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Co-authored-by: David Baker <dbkr@users.noreply.github.com>

test/stores/RoomNotificationStateStore-test.ts

@@ -125,6 +125,7 @@ describe("RoomNotificationStateStore", function () {
         ret.getPendingEvents = jest.fn().mockReturnValue([]);
         ret.isSpaceRoom = jest.fn().mockReturnValue(false);
         ret.getUnreadNotificationCount = jest.fn().mockReturnValue(numUnreads);
+        ret.getRoomUnreadNotificationCount = jest.fn().mockReturnValue(numUnreads);
         return ret;
     }
 });