[MERGE] Handle groups more gracefully in Django

[Stephen Gallagher]

This supersedes the patch I sent earlier today.

I tested this by manually modifying the user database and confirming 
that FAS overrode all settings I made locally, either removing or adding 
group memberships as appropriate.

When the groups are unchanged, we will only require a single lookup into 
the database, as opposed to one per group the user was a member of.

It also addresses a bug where users whose group membership was revoked 
would still appear to be a member within Django apps.

-- 
Stephen Gallagher
RHCE 804006346421761

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