[bug 1068720] Forums per language per product

I still need an admin interface for this, but does this look okay? I had a way more complicated way I was trying to do this and then figured this was probably the best solution. 3 commits for 3 stages of migrations.

f?
See https://github.com/mozilla/kitsune/blob/master/kitsune/wiki/migrations/0003_add_afrikaans.py for example