Fixed #20782 -- qs.values().aggregate() failure

In the combination of .values().aggregate() the aggregate_select_mask
didn't include the aggregates added. This resulted in bogus query.

Thanks to Trac alias debanshuk for report.
Fixed #20788 -- exclude() generated subquery failure

"Fixed" by adding a test case, the original problem was already fixed
by earlier ORM changes. Thanks to david@judicata.com for the report
and test case.
Minor change to get_extra_descriptor_filter()

Refs #20611.
Fixed #19019 -- Fixed UserAdmin to log password change.

Thanks Tuttle for the report.
Fixed #20761 -- Fixed DatabaseError handling in get_or_create and update_or_create.
Fixed #20760 -- Reduced timing variation in ModelBackend.

Thanks jpaglier and erikr.
Fixed #19900 -- Updated admin buttons to use CSS3 rounded corners.