Hook up prepare_pay_inapp to mkt/webpay API (bug 980092)

- Move JWT producers to mkt/webpay
- Create end point in mkt/webpay to retrieve an in app JWT
- Add tests
Merge pull request #1880 from JaredKerim-Mozilla/980092

Hook up prepare_pay_inapp to mkt/webpay API (bug 980092)
fix PR #1883
Merge pull request #1884 from magopian/fix-pr-1883

Fix the PR #1883 (commit a8a6c63) for bug 986221
update legal-docs
Upgrade to Django 1.6.

This reverts commit 2d5bdc17e7c1820acf9c0b24217de558186c7eb0.
fix requirements typo
Changed device types to platforms and form factors

Bugs 969242, 969243, 978150, 978160, 978210