Allow promoting a user to admin via the manage users page.

Sometimes (such as if you're using ldap for authentication),
the primary admin of the install won't be that of the initial
superuser.

Using the initial superuser, you should be able to promote any
other user to be an admin - via the is_staff flag.

This patch allows this promotion, by extending the ChangeUserForm,
currently used to administratively modify users.
Merge pull request #814 from DamianZaremba/make_user_admin

Allow promoting a user to admin via the manage users page.