The migration for the ToS signup field happend in 2014. Everyone who hasn't updated yet needs to live with the English text "Terms of Service". There's no need to keep these unused translations forever.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Since I18n has a DB backend now, I've introduced a helper we can use to
skip overrides in certain situations. Otherwise migration from empty
databases was broken.
