MM-58577 Check remote ownership for posts and reactions (#27317)

* - ensure that posts and reactions can only be added via sync when coming from a remote that the target channel is shared with.
- ensure that posts and reactions are only modified/deleted by the remote that owns them.

* check that reaction belongs to post that belongs to channel that is shared with remote;  check that posts belong to channel shared with remote

* check for correct error type in unit test

* tweak unit test

server/channels/store/timerlayer/timerlayer.go

@@ -6937,6 +6937,22 @@ func (s *TimerLayerReactionStore) GetForPostSince(postId string, since int64, ex
 	return result, err
 }
 
+func (s *TimerLayerReactionStore) GetSingle(userID string, postID string, remoteID string, emojiName string) (*model.Reaction, error) {
+	start := time.Now()
+
+	result, err := s.ReactionStore.GetSingle(userID, postID, remoteID, emojiName)
+
+	elapsed := float64(time.Since(start)) / float64(time.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("ReactionStore.GetSingle", success, elapsed)
+	}
+	return result, err
+}
+
 func (s *TimerLayerReactionStore) GetUniqueCountForPost(postId string) (int, error) {
 	start := time.Now()