Fixed #19743 - Documented some limitations of contrib.auth.

Thanks Aymeric for the suggestion.
Improved regex in strip_tags

Thanks Pablo Recio for the report. Refs #19237.
Fixed #19704 -- Make use of new ungettext_lazy function at appropriate places
Fixed a typo in the test suite that was causing some spurious failures with pypy.
Fixed #15808 -- Added optional HttpOnly flag to the CSRF Cookie.

Thanks Samuel Lavitt for the report and Sascha Peilicke for the patch.
Fixed #19756 - Corrected a ManyToMany example and added some links and markup.
Fixed #19706 - Tweaks to the tutorial.

Thanks Daniele Procida.
Updated createsuperuser to use unicode_literals. Refs #19757.
Fixed #13085 -- Don't fail on creation of model with GFK to a model with __len__() returning zero.

Also, according to the comments on the ticket and its duplicates, added
tests execising saving an instance of a model with a GFK to:

* An unsaved object -- This actually doesn't generate the same failure
  but another ORM-level exception. The test verifies it's the case.

* An instance of a model with a __nonzero__() method thant returns False
  for it. This doesn't fail because that code path isn't executed.

* An instance of a model with a CharField PK and an empty value for it.
  This doesn't fail.
Changed test assertion strategy used in 04d9730.
Fixed #19772 -- Handled APPEND_SLASH correctly in the redirects app.
Typo in i18n docs.
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.