Introduce sticky rooms to the new room list

Originally this was intended to be done only in the importance algorithm, however it is clear that all algorithms will need to deal with this. As such, it has been put into the base class to deal with as we may override it in the future. 

This commit should be self-documenting enough to describe what is going on, though the major highlight is that the handling of the sticky room is done by lying to the underlying algorithm.

This has not been optimized for performance yet.

For https://github.com/vector-im/riot-web/issues/13635

src/stores/room-list/models.ts

@@ -40,4 +40,5 @@ export enum RoomUpdateCause {
     Timeline = "TIMELINE",
     RoomRead = "ROOM_READ", // TODO: Use this.
     NewRoom = "NEW_ROOM",
+    RoomRemoved = "ROOM_REMOVED",
 }