Introduce a TimelineItemThreadSummary object (#4032)

* Introduce a `TimelineItemThreadSummary` object to hold details about threads starting from that particular item

This patch introduces a thread summary object that will be available on main timeline messages that are the root for a given thread.
It currently provides the latest message content and sender for that thread but it will grow to provide info on the number of replies, unreads etc.

It also add a new UI component called `TimelineThreadSummaryView` that makes use of this data and is in turn used by the bubbled styler to render it in the timeline.

The rest of the PR is about refactoring on the `RoomTimelineItemFactory` so that replies and thread summaries use similar paths and builders.

* Add a feature flag for threads

* Address PR comments

* Converge on single implementation for message previews

ElementX/Sources/Other/Avatars.swift

@@ -78,6 +78,7 @@ enum UserAvatarSizeOnScreen {
     case mediaPreviewDetails
     case sendInviteConfirmation
     case sessionVerification
+    case threadSummary
 
     var value: CGFloat {
         switch self {
@@ -119,6 +120,8 @@ enum UserAvatarSizeOnScreen {
             return 64
         case .sessionVerification:
             return 52
+        case .threadSummary:
+            return 28
         }
     }
 }