Reorganize groups module in user admin

[Ben Spaulding]

The groups form widget should be moved into the permissions module.

Groups are permissions tools so it makes sense to have
them inside the permissions module. Having the single group
form widget reside under the groups module (as the only item)
and separated from the permissions module creates visual
noise and may cause confusion.

[Aymeric Augustin]

It may be even better to show the groups before the permissions, since it's a good idea to assign most permissions to groups, and give user permissions only in special cases.

I don't believe this really needs tests — if we have a good reason to change the order of the fields again in the future, we can always check the SVN logs. This page shows that the only previous change to the order of the fields was the fix for #11409.

[Julien Phalip]

In [17325]:

Fixed #11410 -- Moved the "Groups" field to the "Permissions" module in the auth.User admin form and modified the help texts for the user_permissions and groups model fields to reflect that change. Also did a little PEP8 cleanup and improved some docstrings while I was in the area. Thanks to benspaulding and Aymeric Augustin for the suggestions.