Allow admins to add system wide root certificates

apps/files_external/js/settings.js

lib/private/http/client/client.php

lib/private/security/certificatemanager.php

lib/private/server.php

lib/public/icertificatemanager.php

lib/public/iservercontainer.php

settings/admin.php

settings/application.php

settings/controller/certificatecontroller.php

settings/js/certificates.js

settings/js/personal.js

settings/personal.php

settings/routes.php

settings/templates/certificates.php

settings/templates/personal.php

tests/lib/security/certificatemanager.php

tests/settings/controller/CertificateControllerTest.php