Use Pajamas alert in repository mirroring settings

Use Pajamas alert component to display errors
shown when saving invalid values while updating
repository mirroring settings.

Changelog: changed

app/views/admin/application_settings/_repository_mirrors_form.html.haml

 = gitlab_ui_form_for @application_setting, url: repository_admin_application_settings_path(anchor: 'js-mirror-settings') do |f|
-  = form_errors(@application_setting)
+  = form_errors(@application_setting, pajamas_alert: true)
 
   %fieldset
     .form-group