Add feed collection models, serializer, and viewset.
add help text for reviewer comments in submission flow (bug 837809)
remove unused `common/forms.styl` stylesheet
Verify inapp purchase receipts (bug 977767)

- Update verifier to validate contrib on in app purchase
- Update tests to include inapp purchase tests
Merge pull request #2149 from JaredKerim-Mozilla/977767

Verify inapp purchase receipts (bug 977767)
Allow apps to be set with create/update collection views
Add pagination to base feed collection class.
Merge pull request #2141 from chuckharmston/feedcollections

Add feedcollection models, serializer, and viewset.
Fix duplicate migration number
Add docs for feed collections
Merge pull request #2161 from chuckharmston/feedcollection_docs

Add docs for feed collections
add order and item_type to feeditems (feed)
feed builder api view (feed)
update feeditem docs and cleanup
filter feeditem by region/carrier (bug 958305)
Merge pull request #2151 from ngokevin/feeditem

enhance FeedItem for feed builder stuff
allow empty bg color for collections, rename to background_color for cons.
Merge pull request #2162 from ngokevin/collcolor

allow empty bg color for collections, rename to background_color for consistency
update feed coll docs
add collection to feeditem serializer