Mark relationship visible when syncing group with peer

This branch updates the private group invitation protocol to use @grote's new method for marking a contact relationship visible to the group.

I've changed the method slightly because the protocol state machine allows us to leave and re-enter the BOTH_JOINED state (see diagram on #659 (closed)), so the relationship may already be visible when the method is called. In that case the visibility isn't updated, so we stick with whichever of revealed-by-us and revealed-by-contact happened first.