Change inapp public ID to UUID (bug 1035448)

- Add guid field to inapp
- Update APIs to use guid
- Update inapp product pages in devhub to use guid
- Update purchase to use guid
- Update receipts to use guid
- Update tests
Merge pull request #2421 from JaredKerim-Mozilla/1035448

    Change inapp public ID to UUID (bug 1035448)
Update In App Product Documentation to use UUID (bug 1056274)
Merge pull request #2433 from JaredKerim-Mozilla/1056274

Update In App Product Documentation to use UUID (bug 1056274)
forgot premium_type in feed app serializer (bug 1055742)