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/useRoomAvatar.ts

@@ -0,0 +1,26 @@
+/*
+Copyright 2022 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { useCallback } from "react";
+import { Room } from "matrix-js-sdk/src/models/room";
+
+import { useRoomState } from "./useRoomState";
+
+export const useRoomAvatar = (room: Room) =>
+  useRoomState(
+    room,
+    useCallback(() => room.getMxcAvatarUrl(), [room])
+  );