allow admin to disable sharing for specific groups of users

apps/files/css/files.css

apps/files_sharing/js/share.js

apps/files_sharing/lib/permissions.php

apps/files_sharing/lib/sharedstorage.php

apps/files_sharing/tests/api.php

apps/files_sharing/tests/base.php

apps/files_sharing/tests/proxy.php

core/js/config.php

lib/private/files/cache/permissions.php

lib/private/files/storage/common.php

lib/private/share/share.php

lib/private/util.php

lib/public/util.php

settings/admin.php

settings/ajax/excludegroups.php

settings/css/settings.css

settings/js/admin.js

settings/routes.php

settings/templates/admin.php

tests/lib/util.php