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.
Pull 1.7 release notes images into the docs tree.

The images used for the 1.7 release notes were all hosted on reviewboard.org.
This was both annoying in terms of things being self-contained (we'd have to go
upload things separately from writing the docs), and the format of them was
pretty bad when building the docs to a format other than HTML.

I've downloaded all of the images and referenced them locally instead of
remotely. In addition, I've added some conditionals to include the full images
when the target isn't HTML and fixed the latex target.

Testing done:
* Built the HTML docs and verified that the thumbnails showed up, and that
  links to the full versions went to the correct place.
* Built the latex docs, compiled to PDF, and verified that the full screenshots
  were included rather than the thumbnails.

Reviewed at https://reviews.reviewboard.org/r/4646/