Move certificate management interface from files_external to core

apps/files_external/personal.php

apps/files_external/templates/settings.php

lib/private/certificatemanager.php

lib/private/l10n.php

lib/public/icertificatemanager.php

settings/ajax/addRootCertificate.php

settings/ajax/removeRootCertificate.php

settings/css/settings.css

settings/js/personal.js

settings/personal.php

settings/routes.php

settings/templates/personal.php