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.
Fixed test failures introduced in e716518ad29898fb25c820023aaf2fdd1c9eb4af

refs #20761
Fixed #20761 -- Reworded ForeignKey default error message
Fixed #10284 -- ModelFormSet.save(commit=False) no longer deletes objects

Thanks laureline.guerin@ and Wedg.
Fixed #20794 -- Documented changes to validate_email

4e2e8f39d changed the way validate_email behaves for foo@localhost
email addresses, but wasn't listed in the release notes.
Fixed #14300 -- Fixed initial SQL location if models is a package.

Thanks al_the_x for the report and fheinz for the draft patch.
Fixed typo in Custom management commands documentation.
Merge pull request #1395 from funkbit/master

Fixed typo in Custom management commands documentation.
Fixed #19941 -- Modified runtests.py to make running the tests easier.

1. Automatically use tests/../django as the Django version.
2. If settings aren't provided through --settings or DJANGO_SETTINGS_MODULE)
   then use test_sqlite.
Fixed #20766 -- Deprecated FastCGI support.
Fixed #18168 -- clarified precedence of validation

any choices set by formfield_for_choice_field are still subject
to model validation of the model field's choices attribute
Fixed #20792 -- Corrected DISALLOWED_USER_AGENTS docs.

Thanks simonb for the report.
Fixed ._meta.pk_index() virtual field failure
Fixed #18315 -- Documented QueryDict.popitem and QueryDict.pop

Thanks gcbirzan for the report.