Fix back pagination (#432)

* Use pagination and start items for view state.

isBackPaginating and canBackPaginate are updated each time the timeline is rebuilt

* Update some timeline snapshots

The top section has gone, which has altered the layout slightly.

ElementX/Sources/Services/Timeline/TimelineController/RoomTimelineControllerProtocol.swift

@@ -21,8 +21,8 @@ import UIKit
 enum RoomTimelineControllerCallback {
     case updatedTimelineItems
     case updatedTimelineItem(_ itemId: String)
-    case startedBackPaginating
-    case finishedBackPaginating
+    case canBackPaginate(Bool)
+    case isBackPaginating(Bool)
 }
 
 enum RoomTimelineControllerAction {