Remove the 'Category' model, use constants instead (bug 987880)

https://bugzilla.mozilla.org/show_bug.cgi?id=987880

**TODO:**
- [x] Cleanups in forms & validation (though devhub is working fine)
- [x] Fixes for existing tests
- [ ] Migration to convert existing data (for apps, collections, feed items)
I need to come up with the migration for the addons table first (last item on my todo list)