[fix bug 970806] Switch to users.UserProfile instead auth.User in Admin Panel.

Since all of our models that relate to users link to UserProfile, make
the later the entry point for user profile edit in Admin Panel. This
allows us to inline models like GroupMembership which couldn't be
inlined within User.
Merge pull request #842 from glogiotatidis/admin-userprofile

[fix bug 970806] Switch to users.UserProfile instead auth.User in Admin Panel.