Fix translations at load_page plugin

To be translatable text field should be marked
with @i18n. Also these messages should be provided by
i18n_messages.

Fixes: https://pagure.io/freeipa/issue/7559
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Petr Vobornik <pvoborni@redhat.com>

ipaserver/plugins/internal.py

@@ -352,6 +352,7 @@ class i18n_messages(Command):
                 "tickets (obtainable via kinit) and <a href='http://${host}/"
                 "ipa/config/ssbrowser.html'>configured</a> the browser "
                 "correctly, then click 'Log in'."),
+            "loading": _("Loading"),
             "loading_md": _("Loading data"),
             "login": _("Log in"),
             "login_certificate": _("Log In Using Certificate"),