Fix 'cannot find room' error

We weren't waiting for rooms to arrive down the sync stream after
joining them but before trying to use them.

More regression details in linked issue.

Fixes https://github.com/vector-im/element-call/issues/477

src/matrix-utils.ts

@@ -220,8 +220,8 @@ export function isLocalRoomId(roomId: string): boolean {
 export async function createRoom(
   client: MatrixClient,
   name: string
-): Promise<string> {
-  await client.createRoom({
+): Promise<[string, string]> {
+  const result = await client.createRoom({
     visibility: Visibility.Private,
     preset: Preset.PublicChat,
     name,
@@ -251,7 +251,7 @@ export async function createRoom(
     },
   });
 
-  return fullAliasFromRoomName(name, client);
+  return [fullAliasFromRoomName(name, client), result.room_id];
 }
 
 export function getRoomUrl(roomId: string): string {