Use a readable name for the auto-generated site groups and no longer filter auto-generated groups in the User admin.

When a site's domain or a role name is changed, the site group's names get updated as well.

This has many advantages:
* using the auth.User Change User view a superuser can easily see all the sites a user has roles on
* seeing users not being assigned to any groups was kind of confusing (when they were actually assigned to auto-generated site groups)

Also renamed a couple of signal handlers and role methods.
Add migration for renaming the already existing autogenerated site groups
Merge pull request #15 from pbs/readable-site-group-names

Use a readable name for the auto-generated site groups and no longer filter auto-generated groups in the User admin.