server to server sharing next generation

Next step in server-to-server sharing next generation, see https://github.com/owncloud/core/issues/12285

Beside some small improvements and bug fixes this will probably the final state for OC8.

To test this you need to set up two ownCloud instances. Let's say:
- URL: myPC/firstOwnCloud user: user1
- URL: myPC/secondOwnCloud user: user2

Now user1 can share a file with user2 by entering the username and the URL to the second ownCloud to the share-drop-down, in this case "user2@myPC/secondOwnCloud".

The next time user2 login he will get a notification that he received a server-to-server share with the option to accept/decline it. If he accept it the share will be mounted. In both cases a event will be send back to user1 and add a notification to the activity stream that the share was accepted/declined.

If user1 decides to unshare the file again from user2 the share will automatically be removed from the second ownCloud server and user2 will see a notification in his activity stream that user1@myPC/firstOwnCloud has unshared the file/folder from him.

ToDo (I will provide PRs for it):

- [ ] add implementation  isShareTypeAllowed() in share_backend for calendar
- [ ] add implementation  isShareTypeAllowed() in share_backend for contacts

This is ready for testing and reviewing cc @LukasReschke @icewind1991 @MorrisJobke @PVince81 
Always string?