Allow network admins to promote users on their network

Network admins were previously able to add existing users to a site
through the site edit screen at the network level. They were not
able to add existing network users to a site at the add user page
at the site level.

Now - network admins can add existing users at the site level.
Merge pull request #207 from washingtonstateuniversity/bug/network-admin-users

[PR] Allow network admins to promote users on their network