loginDialog: Add an :expanded pseudo class to the user list

loginDialog: Add an :expanded pseudo class to the user list

We want to style user list items differently depending on whether
the list is expanded or shrunk; instead of manually updating the
items' style, we can just expose the :expanded style on the list
itself and use that in the CSS.

https://bugzilla.gnome.org/show_bug.cgi?id=658185