Merge pull request #20786 from owncloud/systemtags-dav

apps/dav/lib/rootcollection.php

apps/dav/lib/server.php

apps/dav/lib/systemtag/systemtagmappingnode.php

apps/dav/lib/systemtag/systemtagnode.php

apps/dav/lib/systemtag/systemtagplugin.php

apps/dav/lib/systemtag/systemtagsbyidcollection.php

apps/dav/lib/systemtag/systemtagsobjectmappingcollection.php

apps/dav/lib/systemtag/systemtagsobjecttypecollection.php

apps/dav/lib/systemtag/systemtagsrelationscollection.php

apps/dav/tests/unit/systemtag/systemtagmappingnode.php

apps/dav/tests/unit/systemtag/systemtagnode.php

apps/dav/tests/unit/systemtag/systemtagplugin.php

apps/dav/tests/unit/systemtag/systemtagsbyidcollection.php

...ests/unit/systemtag/systemtagsobjectmappingcollection.php

...v/tests/unit/systemtag/systemtagsobjecttypecollection.php

lib/private/systemtag/systemtagmanager.php

lib/private/systemtag/systemtagobjectmapper.php

lib/public/systemtag/isystemtagmanager.php

tests/lib/systemtag/systemtagmanagertest.php

tests/lib/systemtag/systemtagobjectmappertest.php