Add system tags filter section for files app

apps/systemtags/appinfo/app.php

apps/systemtags/css/systemtagsfilelist.css

apps/systemtags/img/tag.png

apps/systemtags/img/tag.svg

apps/systemtags/js/app.js

apps/systemtags/js/filesplugin.js

apps/systemtags/js/systemtagsfilelist.js

apps/systemtags/list.php

apps/systemtags/templates/list.php

apps/systemtags/tests/js/systemtagsfilelistSpec.js

core/js/files/client.js

core/js/systemtags/systemtagsinputfield.js

core/js/tests/specs/files/clientSpec.js

tests/karma.config.js