Remove `AddonType` model and everything linked to it (bug 1026178)

https://bugzilla.mozilla.org/show_bug.cgi?id=1026178
We don't, because the user is already set as an app author for this app. I was starting to get failures because of this, so I fixed them as well. I suspect we had a bug about this for a while but didn't see it till I started messing with the fixtures in this PR.