Add LocalSite validation for Group's list of users.

Review Request #2041 — Created Jan. 15, 2011 and submitted — Latest diff uploaded

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Add LocalSite validation for Group's list of users.

This introduces new validation for Group that ensures the users selected
are actually part of the LocalSite, if the Group is a part of one.

This makes use of the same user validation logic as DefaultReviewer. The
checks have been split out into a _validate_users function, which both
can access.

Unit tests have been added to ensure that validation works how we expect.