Add a link to the latest diff revision in the review request resource

Review Request #8270 — Created July 6, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
9c5f21c...

Reviewers

The review request resource now links to the latest diff (as
latest_diff in the links section of the response) when the
associated review request has diffs.

Ran unit tests.