Update admin page for users.

I've been running out of space on the admin page for users.

I've removed the IP address and locked columns from the admin page for
users. The IP address field is unneccesary, since most logins will be on
a University of Oxford IP address. I doubt users will become locked, so
I've also removed that column. Locking and unlocking users can still be
done on the edit page, for individual users.
Add "add user" function to admin page.

Admins can now add users. I've added both `new' and `create' actions to
the admin user controller, as well as added a view, which is basically
the same as the Devise `new registration' view. Routes were unchanged,
since there was already a path for `new_admin_user', as well as one
handling a post to `admin_users'.