Changes for Firefox OS newsletter launch

Note to @pmclanahan: We're still pending confirmation from Jess that the behavior in this PR is exactly what we want, so not ready to merge. This implements the latest design we've discussed, as I understand it.

I've refactored some of the code to make the logic clearer, so please check this out.

- Check both Master_Subscribers and OPT_IN before concluding a user
  does not exist in Exact Target
- Make more explicit behavior on subscribing in various
  cases
- Add support for translated welcome messages
- More tests
- Other refactoring and cleanup