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.