Removed u'' prefixes which are no longer needed on Python 3.
Added Django docs to intersphinx and a ton of crosslinks.

To make this work, conf.py includes some fake crosslink roles like
"django-admin", "setting" and "templatetag". As not all labels are
obvious, you can lookup the correct links in "django_1_6_inv.txt".

Also fixed some parts which were hard to follow or placed
in code blocks.