Another pass at improvements to FOIA request intake API

[konklone]

I opened a discussion thread on our API (to make sure non-gov't people saw it) at usdoj/foia.gov#25, and included some thoughts on next steps. They're not complete, and I'd encourage discussion and additions to be added to that thread.

As an actionable thing, let's group some of the immediate changes I outlined, assuming they're non-objectionable, on this ticket. We're not building to a committed v1 here -- this is another round of improvements to make this feel more like a proper API.

So this includes:

• Field name changes
• Ensuring optional and required fields in the API match those as indicated on form
• Cleaning up response body