Fix external_id on products

To match the in-app payments model, external IDs
can be made up by sellers so they aren't globally unique.
Consistency across active statuses in transactions
Merge pull request #37 from davidbgk/master

Consistency across active statuses in transactions
GET/filter products (bug 939996)
Merge pull request #38 from kumar303/get-products

GET/filter products (bug 939996)
Make products curling compatible (bug 939996)
Adding the ability to get terms with a date (bug 932577)

https://bugzilla.mozilla.org/show_bug.cgi?id=932577

The date is optional and no logic is performed on the zippy side, the
client have the responsibility to deal with the validity/expiry of
terms given that date.
Merge branch 'master' of https://github.com/mozilla/zippy into 932577-adding-terms
Merge pull request #39 from davidbgk/932577-adding-terms

Adding the ability to get terms with a date (bug 932577)
Make filtering by nonexistant seller more explicit
Initial form styling + cc page

Initial form styling + cc page