Create a start in-app payment transaction API (bug 944519)

- Add In-App JWT producer
- Expose JWT in public view
- Add tests

This is in an intermediate state.  Presently it implements two entirely separate token producers, one for in app and one for web app, which share a lot of logic, so there's duplication in this code.  I've created https://bugzilla.mozilla.org/show_bug.cgi?id=979551 to handle refactoring this.