MM-34609 Mark-as-unread on a post in a thread should cause auto-follow (#17343)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/opentracinglayer/opentracinglayer.go

@@ -8550,7 +8550,7 @@ func (s *OpenTracingLayerThreadStore) GetThreadsForUser(userId string, teamID st
 	return result, err
 }
 
-func (s *OpenTracingLayerThreadStore) MaintainMembership(userID string, postID string, following bool, incrementMentions bool, updateFollowing bool, updateViewedTimestamp bool) error {
+func (s *OpenTracingLayerThreadStore) MaintainMembership(userID string, postID string, following bool, incrementMentions bool, updateFollowing bool, updateViewedTimestamp bool) (*model.ThreadMembership, error) {
 	origCtx := s.Root.Store.Context()
 	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "ThreadStore.MaintainMembership")
 	s.Root.Store.SetContext(newCtx)
@@ -8559,13 +8559,13 @@ func (s *OpenTracingLayerThreadStore) MaintainMembership(userID string, postID s
 	}()
 
 	defer span.Finish()
-	err := s.ThreadStore.MaintainMembership(userID, postID, following, incrementMentions, updateFollowing, updateViewedTimestamp)
+	result, err := s.ThreadStore.MaintainMembership(userID, postID, following, incrementMentions, updateFollowing, updateViewedTimestamp)
 	if err != nil {
 		span.LogFields(spanlog.Error(err))
 		ext.Error.Set(span, true)
 	}
 
-	return err
+	return result, err
 }
 
 func (s *OpenTracingLayerThreadStore) MarkAllAsRead(userID string, teamID string) error {