Bug 886715 - [MMS] [Regression] After changing language all strings should be in new language

This is a complete overhaul of all localization inside the SMS app.

- Avoid using `navigator.mozL10n.get()` to populate `innerText` ourselves
  - Exceptions to this rule: when the final target is something like `alert()` or `navigator.mozNotification`, we can't really pass HTML, so do our best with the current translated string
- Use `navigator.mozL10n.localize(element, id, args)` where possible
- Use `navigator.mozL10n.translate(fragment)` and stuff the proper data-* attribs where we render templates.