Create a JWT for a navigator.pay() app payment, signed with the developer's secret. This is the backend to support just the JWT part for the frontend (bug 776638). It doesn't cover postbacks. Spec: https://wiki.mozilla.org/WebAPI/WebPayment#Marketplace_payment
since solitude holds seller/developer info, the JWT should probably be generated by solitude. Marketplace can call something like: solitude.get_bluevia_app_payment_jwt(app.pk, mkt_postback_url, mkt_chargeback_url)
I landed backend stuff that the UI can use: https://github.com/mozilla/zamboni/commit/04aa011e1c5974ca7f9c94ad64f12606efbbe0d3 This needs some more work (in other bugs) and also will make calls to non-existant solitude APIs. If needed, we can mock solitude until it's ready.