We recently re-worked our ReviewRequestPageData to reduce the amount of

queries we were doing. In that change, we added some processing for diff

comments where we attach pre-loaded diffsets back to the comments.

However, we weren't pre-loading the draft diffset of the review request,

and didn't consider the draft diffset when processing the comments.
So if a user was viewing a draft review request and they added a draft

diff comment, this would cause the comment processing to crash on the next

page load. This change includes the draft diffset in our list of loaded

diffsets on the ReviewRequestPageData if the draft is accessible by the

requesting user, allowing the draft diffset to be attached to comments

during processing.

Ran unit tests.
Viewed a draft review request that had a draft diffset on it. Created

  a diff comment and hard reloaded the page. Made sure the page didn't

  crash, whereas previously it would crash with a key error in

diffsets_by_id.
Viewed a review request that has a draft as an anonymous user. Made

  sure that I couldn't see the draft, and couldn't see any draft diff

  comments.
Created a draft review request that had a draft diffset and draft

  comment on it. Viewed the draft review requets as a superuser/admin

  that does not own the draft comment/review, made sure that the admin

  could not see the draft diff comment or review.