l10n SMS messages

Find a way to localize SMS messages.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1024383
`i18n-abide` also exposes `req.format` to help with interpolation:

```
  message = req.gettext("Your verification code is: %(code)s", {
    code: code
  });
```

`%(code)s` might be a little friendlier to localizers than `%s`.