Allow expressing more explicitly the direction in which an entity replication is allowed

There is currently a readonly property in the replication configuration (but not available in the UI) allowing to indicate that an instance when send an update about a document and the target instance is not allowed to modify that document.

It would make things a little clearer to express this use case as directions a replication is allowed to take: both, only send, only receive so that it's easier to understand and manipulate for custom replication controllers.