Add API for working with drafts of diffs.
There was no way, through the API, to get an existing diff uploaded as a

draft. One could be created by issuing a POST, and the payload would be

returned, but the links to the resource in that payload would result in

a 404 due to the diffs resource not showing draft diffs.
This fixes this by introducing a new

/api/review-requests/<id>/draft/diffs/... tree that mirrors the public

/api/review-requests/<id>/diffs/... tree. Diffs that are part of a draft

will link there. The client can inspect this to see everything that will

be published.

Unit tests pass.
Tested uploading a diff against the local server and saw that resulting

payload was correct and linked to the correct place.