Working UI for external storage mount configuration

apps/files_external/ajax/addMountPoint.php

apps/files_external/ajax/removeMountPoint.php

apps/files_external/appinfo/app.php

apps/files_external/css/settings.css

apps/files_external/js/settings.js

apps/files_external/lib/config.php

apps/files_external/personal.php

apps/files_external/settings.php

apps/files_external/templates/settings.php