message: Use the correct timeline reference for message previews
  Fixes (hopefully)
  This is the same logic used by `Unread.js`, so should be correct.
diff: @@ -78,9 +78,8 @@ export class MessagePreviewStore extends AsyncStoreWithClient<IState> {
  }
  private generatePreview(room: Room) {
  - const timeline = room.getLiveTimeline();
  - if (!timeline) return; // usually only happens in tests
  - const events = timeline.getEvents();
  + const events = room.timeline;
  + if (!events) return; // should only happen in tests
  for (let i = events.length - 1; i >= 0; i--) {
  if (i === events.length - MAX_EVENTS_BACKWARDS) return; // limit reached
  