Revise localized agreements

This is a very rough draft of revising the way we're localizing the agreements after the discussion following https://github.com/mozilla/fxoss/issues/199#issuecomment-51694614. Will utilize the "Ready for Review" label when appropriate.
Instead of assuming this is English, it'd be better to just look for an agreement in `settings.LANGUAGE_CODE` and 404 if one cannot be found.