MM-34758 Collapsed Reply Threads without mobile support (#17424)

Summary
added support for legacy clients accessing server
added collapsed_threads_supported param to viewChannel API and setPostUnread API

Ticket Link
https://mattermost.atlassian.net/browse/MM-34758

Related Webapp PR
mattermost/mattermost-webapp#7933

store/opentracinglayer/opentracinglayer.go

@@ -2176,7 +2176,7 @@ func (s *OpenTracingLayerChannelStore) UpdateLastViewedAt(channelIds []string, u
 	return result, err
 }
 
-func (s *OpenTracingLayerChannelStore) UpdateLastViewedAtPost(unreadPost *model.Post, userID string, mentionCount int, mentionCountRoot int, updateThreads bool) (*model.ChannelUnreadAt, error) {
+func (s *OpenTracingLayerChannelStore) UpdateLastViewedAtPost(unreadPost *model.Post, userID string, mentionCount int, mentionCountRoot int, updateThreads bool, setUnreadCountRoot bool) (*model.ChannelUnreadAt, error) {
 	origCtx := s.Root.Store.Context()
 	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "ChannelStore.UpdateLastViewedAtPost")
 	s.Root.Store.SetContext(newCtx)
@@ -2185,7 +2185,7 @@ func (s *OpenTracingLayerChannelStore) UpdateLastViewedAtPost(unreadPost *model.
 	}()
 
 	defer span.Finish()
-	result, err := s.ChannelStore.UpdateLastViewedAtPost(unreadPost, userID, mentionCount, mentionCountRoot, updateThreads)
+	result, err := s.ChannelStore.UpdateLastViewedAtPost(unreadPost, userID, mentionCount, mentionCountRoot, updateThreads, setUnreadCountRoot)
 	if err != nil {
 		span.LogFields(spanlog.Error(err))
 		ext.Error.Set(span, true)
@@ -8938,6 +8938,24 @@ func (s *OpenTracingLayerThreadStore) MarkAllAsRead(userID string, teamID string
 	return err
 }
 
+func (s *OpenTracingLayerThreadStore) MarkAllAsReadInChannels(userID string, channelIDs []string) error {
+	origCtx := s.Root.Store.Context()
+	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "ThreadStore.MarkAllAsReadInChannels")
+	s.Root.Store.SetContext(newCtx)
+	defer func() {
+		s.Root.Store.SetContext(origCtx)
+	}()
+
+	defer span.Finish()
+	err := s.ThreadStore.MarkAllAsReadInChannels(userID, channelIDs)
+	if err != nil {
+		span.LogFields(spanlog.Error(err))
+		ext.Error.Set(span, true)
+	}
+
+	return err
+}
+
 func (s *OpenTracingLayerThreadStore) MarkAsRead(userID string, threadID string, timestamp int64) error {
 	origCtx := s.Root.Store.Context()
 	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "ThreadStore.MarkAsRead")