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/stores/LifecycleStore.js

@@ -63,6 +63,7 @@ class LifecycleStore extends Store {
                 dis.dispatch(deferredAction);
                 break;
             }
+            case 'on_client_not_viable':
             case 'on_logged_out':
                 this.reset();
                 break;