Fix vector-im/riot-web#4526 by pretending to join

I thought about adding separate dispatches to prevent confusion but if anyone adds anything that listens to existing dispatches, they really ought to be grep-ing the world for said dispatch actions.

src/stores/RoomViewStore.js

@@ -141,6 +141,10 @@ class RoomViewStore extends Store {
                 shouldPeek: payload.should_peek === undefined ? true : payload.should_peek,
             };
 
+            if (payload.joined) {
+                newState.joining = false;
+            }
+
             // If an event ID wasn't specified, default to the one saved for this room
             // via update_scroll_state. Assume initialEventPixelOffset should be set.
             if (!newState.initialEventId) {