Fixed #19662 -- alter auth modelbackend to accept custom username fields

Thanks to Aymeric and Carl for the review.
Moved import in db.backends.creation
Fixed #19779 -- Checked contrib.sites presence in RedirectFallbackMiddleware

Thanks Aymeric Augustin for the report and directions for the patch.
Removed unneeded imports in db.models init
Cleaned up some lingering signals in the test suite that were causing spurious failures with Pypy and Postgres.
Fixed #18906 -- Ignored to-be-deleted forms in formset validate_unique

Thanks c.pollock at bangor.ac.uk for the report.
Fixed #19749 -- Documented ending param to command's self.stdout/err

Thanks xian at mintchaos.com for the report.
Fixed #19699 - Removed "Please see the release notes" from versionadded/changed directives

Also bumped django_next_version back to 1.6 so those
annotations are described as "the development version"
in the docs.

Thanks Aymeric for the patch.
Added tests for some sql_* management commands
Made modwsgi groups_for_user consistent with check_password

2b5f848207b1dab35afd6f63d0107629c76d4d9a based its changes on #19061
that made the is_active attribute mandatory for user models.
The try/except was not removed for the groups_for_user function.

refs #19780
Adds csrf (failing) tests that has a port number in the Host Req Header
Adds fix for same_origin check where \'good_refer\' has port set to None due to the fact that the HTTP_REFERER usually do not have port set
Merge branch 'master' of github.com:jtinfors/django
Adds a test for csrf with the port part of HTTP_HOST eader set to 80