Show the room avatar rather than the user avatar in the header

It's supposed to be the room avatar; this was a regression introduced during the LiveKit hack week.

src/room/VideoPreview.tsx

@@ -35,6 +35,7 @@ export type MatrixInfo = {
   avatarUrl: string;
   roomName: string;
   roomIdOrAlias: string;
+  roomAvatarUrl: string | null;
 };
 
 interface Props {