Merge pull request #20650 from owncloud/systemtags-core

db_structure.xml

lib/private/server.php

lib/private/systemtag/systemtag.php

lib/private/systemtag/systemtagmanager.php

lib/private/systemtag/systemtagobjectmapper.php

lib/public/iservercontainer.php

lib/public/systemtag/isystemtag.php

lib/public/systemtag/isystemtagmanager.php

lib/public/systemtag/isystemtagobjectmapper.php

tests/lib/systemtag/systemtagmanagertest.php

tests/lib/systemtag/systemtagobjectmappertest.php