Stop migrating to MSC4278 if the config exists. (#29924)

* Stop migrationg to MSC4278 if the config exists.

* Run migration after we have synced the client.

* Setup the SettingsController with a client.

* Add tests to check migration behaviour.

* update copyright

* Wait for sync properly

* Catch failure

* Docs

* licence

* Inline async code

* Fix migrateURLPreviewsE2EE too

* drop an import

* go away

test/test-utils/test-utils.ts

@@ -164,6 +164,7 @@ export function createTestClient(): MatrixClient {
         getVisibleRooms: jest.fn().mockReturnValue([]),
         loginFlows: jest.fn(),
         on: eventEmitter.on.bind(eventEmitter),
+        once: eventEmitter.once.bind(eventEmitter),
         off: eventEmitter.off.bind(eventEmitter),
         removeListener: eventEmitter.removeListener.bind(eventEmitter),
         emit: eventEmitter.emit.bind(eventEmitter),