Make sure the extra translation queries follow db routing (bug 905887)

https://bugzilla.mozilla.org/show_bug.cgi?id=905887

This should fix translations being empty for a small amount of time when creating a collection or an app, at least when creating using the API (since it pins its queries to master when authenticated)

A later fix would be to send the db currently used to the transformers to make them follow that, but that requires a django-queryset-transform patch.

**Edit:** bonus read, how translations work in Zamboni : https://gist.github.com/diox/e99d74a8e445032b49fa