Import slugify from django.utils.text rather than django.templates.

Hopefully this will have no effect at all, but it's slightly neater.
The slugify function is now available from django.utils.text - this
gives you just the function rather than one decorated as a template
filter, and is now the recommended place to get it.

I've also reordered a few other imports to make them more consistent.

A couple of tests needed updating as the import to slugify should always
be in unicode.
Tidy up some odd whitespace.

Mostly deleting trailing whitespace, but a little bit of replacing tabs
with spaces and removing spaces in assignments and function application.
No need to load url from the future - we're in the future!
Add ALLOWED_HOSTS setting.

Add the ALLOWED_HOSTS setting required for production Django instances
from Django 1.5. I'm doing this from the conf/general.yml file so that
we can vary it for different vhosts easily on our dev/staging sites.

Also replace calls to request.META.HTTP_HOST with request.get_host
so as to avoid bypassing this security measure. See

https://docs.djangoproject.com/en/1.5/ref/settings/#allowed-hosts

for details.