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/retrylayer/retrylayer.go

@@ -8741,6 +8741,27 @@ func (s *RetryLayerReactionStore) GetForPostSince(postId string, since int64, ex
 
 }
 
+func (s *RetryLayerReactionStore) GetSingle(userID string, postID string, remoteID string, emojiName string) (*model.Reaction, error) {
+
+	tries := 0
+	for {
+		result, err := s.ReactionStore.GetSingle(userID, postID, remoteID, emojiName)
+		if err == nil {
+			return result, nil
+		}
+		if !isRepeatableError(err) {
+			return result, err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return result, err
+		}
+		timepkg.Sleep(100 * timepkg.Millisecond)
+	}
+
+}
+
 func (s *RetryLayerReactionStore) GetUniqueCountForPost(postId string) (int, error) {
 
 	tries := 0