loginDialog: display banner message when disable-user-list=true

loginDialog: display banner message when disable-user-list=true

The login screen supports showing a banner message which admins
can use to mention login rules or disclaimers.

This message only shows up currently if the user list is enabled.
Most people who want to show a banner message also want to disable
the user list.

This commit moves the banner message to display when the user is
prompted for login credentials instead of when showing the user
list. It also adds a scrollbar if the message is too long.

https://bugzilla.gnome.org/show_bug.cgi?id=703972