user-accounts: Introduce an empty state page

There are eventually corner cases where there's no user other than
root in the system and control-center can be launched by the root
user (not recommended). In this cases, the panel crashes without
any user to show (since accountsservice would just list normal
users).

This patch introduces an empty state[0] "No Users Page".

[0] https://wiki.gnome.org/Design/OS/EmptyStates

https://bugzilla.gnome.org/show_bug.cgi?id=773673