Fix for #1622: don't allow multiple groups with the same name.
General cleanup of code and comments to follow Django standards a bit more carefully.
Fix for #1839: Clean up facility_user views logic to avoid weird errors on failed POST.
Fix for #1838: only validate passwords when it's a new user (no password), or either of the password fields is non-empty.
Fix for #1885: warn if first/last name combo exists already; allow user to override.
Change for #1885: switch from facility-uniqueness check to zone
For security purposes, only show details on existing usernames to admins.