Bug 863066: Changes for Firefox OS newsletter launch

Note to @pmclanahan: We've settled the design and this PR implements it, as far as I can tell. Ready to review.

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

Bug 863066