Clean Ups

Just cleaning up some stuff in ContentManagement and SiteManagement.
Users now need to be is_staff to be logged in; necessary for the CMS.
CMS now checks for whether a user belongs to certain groups like
Administrators, Editors, Authors; Authors is actually redundant
because as far as the CMS is concerned, all is_staff users may be
authors. But then again, we might want to change that.

Also, some new helper functions in ContentManagement and helpers.py.
Hopefully, they will reduce some work when we get to the attachments,
users, and events management stuff.