Fix notifications appearing for old events (#3946)

A method that we use for fetching recursive related events on homeservers
without MSC3981 support injects events into the timeline in timestamp
order using a special method on event-timeline-set. Injecting events using
this method could cause on-screen notifications because it incorrectly set
the 'liveEvent' flag to true if the events were added tio the live timeline.
These events are never live though as the point is that we're fetching them.

src/models/event-timeline-set.ts

@@ -839,7 +839,10 @@ export class EventTimelineSet extends TypedEventEmitter<EmittedEvents, EventTime
 
         const data: IRoomTimelineData = {
             timeline: timeline,
-            liveEvent: timeline == this.liveTimeline,
+            // The purpose of this method is inserting events in the middle of the
+            // timeline, so the events are, by definition, not live (whether or not
+            // we're adding them to the live timeline).
+            liveEvent: false,
         };
         this.emit(RoomEvent.Timeline, event, this.room, false, false, data);
     }