Merge pull request #20355 from owncloud/add-carddav-sharing

apps/dav/appinfo/database.xml

apps/dav/lib/carddav/addressbook.php

apps/dav/lib/carddav/addressbookroot.php

apps/dav/lib/carddav/carddavbackend.php

apps/dav/lib/carddav/sharing/ishareableaddressbook.php

apps/dav/lib/carddav/sharing/plugin.php

apps/dav/lib/carddav/useraddressbooks.php

apps/dav/lib/connector/sabre/auth.php

apps/dav/lib/rootcollection.php

apps/dav/lib/server.php

apps/dav/tests/misc/sharing.xml

apps/dav/tests/unit/carddav/carddavbackendtest.php