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/Screens/RoomScreen/View/TimelineTableViewController.swift

@@ -61,6 +61,9 @@ class TimelineTableViewController: UIViewController {
         }
     }
     
+    /// Whether or not the timeline has more messages to back paginate.
+    var canBackPaginate = true
+    
     /// Whether or not the timeline is waiting for more messages to be added to the top.
     var isBackPaginating = false {
         didSet {
@@ -334,7 +337,8 @@ class TimelineTableViewController: UIViewController {
     ///
     /// Prefer not to call this directly, instead using ``paginateBackwardsPublisher`` to throttle requests.
     private func paginateBackwardsIfNeeded() {
-        guard !isBackPaginating,
+        guard canBackPaginate,
+              !isBackPaginating,
               !hasPendingUpdates,
               tableView.contentOffset.y < tableView.visibleSize.height * 2.0
         else { return }

ElementX/Sources/Screens/RoomScreen/View/TimelineView.swift

@@ -59,6 +59,9 @@ struct TimelineView: UIViewControllerRepresentable {
             if tableViewController.timelineItems != context.viewState.items {
                 tableViewController.timelineItems = context.viewState.items
             }
+            if tableViewController.canBackPaginate != context.viewState.canBackPaginate {
+                tableViewController.canBackPaginate = context.viewState.canBackPaginate
+            }
             if tableViewController.isBackPaginating != context.viewState.isBackPaginating {
                 tableViewController.isBackPaginating = context.viewState.isBackPaginating
             }