Implement basic soft logout handling

Fixes https://github.com/vector-im/riot-web/issues/10235

CSS and copy are left as an exercise for a later iteration.

Login page handling is left for https://github.com/vector-im/riot-web/issues/10236

This implementation reuses as much of the Lifecycle flow as it can without causing problems. Most importantly, it requires https://github.com/matrix-org/matrix-js-sdk/pull/975 to be able to detect a soft logout and react to it. When it comes time to starting/stopping the Lifecycle, additional parameters are provided so that the auxiliary services can (re)start themselves without the client starting to sync.

src/BasePlatform.js

@@ -36,6 +36,7 @@ export default class BasePlatform {
 
     _onAction(payload: Object) {
         switch (payload.action) {
+            case 'on_client_not_viable':
             case 'on_logged_out':
                 this.setNotificationCount(0);
                 break;