Next step in server-to-server sharing next generation, see #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.
Merge pull request #12749 from owncloud/server2server-sharing-ng

server to server sharing next generation