[1.6.x] Fixed link text for "Testing in Django" in tutorial 5.

Backport of e1cfc4b96a from master
[1.6.x] Fixed #21614 -- improved docs on configuring email for development.

Backport of d599b590eb from master
[1.6.x] Fixed #21539 -- Added example of modelformset_factory's form argument

Backport of 1fa681ee11 from master
[1.6.x] Added internal link to i18n documentation.

Backport of e904d421d06bb53fbc9f55bd129b0b213d9908c5 from master.