Add authentication buttons for all enabled strategies to admin UI

Add authentication buttons for all enabled strategies to admin UI, as we are getting more reports from tenant admins saying that they can't login to the admin UI.

The buttons should be styled the same way as the buttons in the topnavigation widget, and should show on top of the local authentication form (divided by a horizontal line with the same style as vertical divider in the topnavigation widget). The local authentication form can also be hidden when local authentication is not enabled.
Going to prefix this with `#adminlogin-external` so its use is clearer