Share tags wip

Okay, I guess to get this reviewed, I need to formally submit it as a PR, even though it's not entirely finished yet.

So here's my work-in-progress branch for owncloud/core#3812. Tested with contacts only, but nothing contacts-specific in the code, so it should work with other apps that use tags as well.

Known issues:

* If the owner and the sharee both have a tag of the same name, there will probably some collision with unknown outcome (untested).
* If the sharee tags a contact of their own with a shared tag, that contact will be counted in the owner's tagged items count as well, i.e. in the number right to the tag's name in the left column of the Contacts app.