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/store.go

@@ -740,6 +740,7 @@ type ReactionStore interface {
 	ExistsOnPost(postId string, emojiName string) (bool, error)
 	DeleteAllWithEmojiName(emojiName string) error
 	BulkGetForPosts(postIds []string) ([]*model.Reaction, error)
+	GetSingle(userID, postID, remoteID, emojiName string) (*model.Reaction, error)
 	DeleteOrphanedRowsByIds(r *model.RetentionIdsForDeletion) error
 	PermanentDeleteBatch(endTime int64, limit int64) (int64, error)
 	PermanentDeleteByUser(userID string) error