Add Enterprise Login dialog
This will split off the Enterprise Login options from the AddUserDialog to a separate dialog as per the mockups at https://gitlab.gnome.org/Teams/Design/settings-mockups/-/blob/master/users/users.png https://gitlab.gnome.org/Teams/Design/settings-mockups/-/blob/master/users/add-enterprise-login.png
This depends on !2090 (merged) that I want to land first to make it easier to review.