[bug 1018727] Backfill translations

This adds a "backfill translations" form to the admin allowing us to
backfill missing translations for responses. It's rough, but probably
good enough for now.

r?
Is the intention to be inclusive or exclusive of the end date? Right now it seems like it goes up to date_end @ 0:00:00 (inclusive). So it excludes most of the end date. (I'm assuming created is a datetime).