Added the Migration.atomic attribute which can be set to False
for non-atomic migrations.

Thanks to Tim Graham for the review.

After a user logs out via auth.views.logout(), they're redirected
to LOGOUT_REDIRECT_URL if no `next_page` argument is provided.

"Gypsy" is considered a slur by the Romani people whom it refers to.
"manouche jazz" is used in place of "gypsy jazz" and is an accepted
term for the same genre of music.

Thanks Berker Peksag for the review.

Allowed admin POSTed bulk-edit data to use modeladmin.get_queryset()
so that the ids in the POST data have a chance to match up even if
the objects on the current page changed based on the ordering.

This is a security fix.

Thanks Ali Lozano for the report and the initial patch, and Tim Graham for
the review.

Thanks František Malina for the report.