Add 'site' foreign key relations to models

We want to be able to associate actions and 'following' with specific
sites rather than having them global.

This is a breaking change as it requires use of the site system. We
might be able to able null's in this field in order to make the support
optional. I'm not sure if that is for the best.

The django comments framework simply requires 'sites' and so does
'django-allauth' so there is some precedent for requiring it even for
people who have single sites.
Add site data to action & 'following' creation & queries

We use the Site.objects.get_current() helper to get the current site.
I'm not sure if this is completely necessary or as effecient as
possible. It might be that settings.SITE_ID will suffice in some
situations.
Add migration for site fields

We add the site fields and set their default to 1 so that for anyone
with a single site, it should just work.
Merge branch 'sites'