Replace is_staff with is_superuser

[vad]

With Impostor i'm able to login as another user, as a superuser too, right? Thus there's no reason to let it available to the staff instead of the restricted superusers "group".

On the other hand, i need to give access to the admin site to translators and content managers, but i don't want to give them super powers :)

[samastur]

Excellent point. I'm kind of swamped right now, but I will try to get this done over this weekend.

[vad]

thanks!

[samastur]

Done. I also added support for IMPOSTOR_GROUP. Users added to that group can impersonate other users who are not superusers.

This should be used cautiously since it still allows impersonating somebody with different permissions.

[vad]

well done! :) thank you!