Fixed #17061 -- Factored out importing object from a dotted path

Thanks Carl Meyer for the report.
Fixed error message test assertions under Python 3.3

Thanks Florian Apolloner for testing.
Fixed #19734 -- Missing values in `DATETIME_INPUT_FORMATS` doc.

Also changed formating of `DATE_INPUT_FORMATS` and
`TIME_INPUT_FORMATS` for readability.

Thanks minddust for the report!
Fixed #19729 -- Removed leftover refactoring helper variables.

Thanks chrismedrela for the report.

Refs #6262, 44b9076 and 4d94c0c.
Fixed #19689 -- Renamed `Model._meta.module_name` to `model_name`.
Fixed #17683 -- Make sure `BaseModelFormSet` respects defined widgets.
Fixed a documentation warning introduced by 3f1c7b7
Fixed #19745 -- Forced resolution of verbose names in createsupersuser

Thanks Baptiste Mispelon for the report and Preston Holmes for the review.
Fixed #9800 -- Allow "isPermaLink" attribute in <guid> element of an RSS item.

Thanks @rtnpro for the patch!
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.