Add encryption history banner and update session verification banner strings. (#1250)

* Add a hard wall on back-pagination in encrypted rooms.
Until the app supports key backup, the history will never be decryptable.

* Fix initial display of the timeline table view.

* Update session verification banner strings.

ElementX/Sources/Services/Timeline/TimelineItems/RoomTimelineItemView.swift

@@ -55,6 +55,8 @@ struct RoomTimelineItemView: View {
             UnsupportedRoomTimelineView(timelineItem: item)
         case .timelineStart(let item):
             TimelineStartRoomTimelineView(timelineItem: item)
+        case .encryptedHistory(let item):
+            EncryptedHistoryRoomTimelineView(timelineItem: item)
         case .state(let item):
             StateRoomTimelineView(timelineItem: item)
         case .group(let item):