[#66999486 #66999576] Create FeedItemsController

Also includes the Serializer, the routes, the spec, and an empty
factory.
[#66999486] Wrote tests and got factories working
[Fix #66999486] Finished FeedItemsController tests
Fix linting errors
Move output generation to the FeedItemsSerializer

This moves the output generation out of the controller which makes it a bit easier to test.

This new serializer combines the output from the FeedItemSerializer and
an array of bad feed ids.