Add an API for commenting on FileDiffs ranges

Review Request #10190 — Created Oct. 19, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
b959551...

Reviewers

The ReviewDiffCommentResource is now capable of creating comments on
FileDiffs with a base FileDiff ID. This allows us to comment on
FileDiffs in commit ranges and keep the context of the base FileDiff.

Ran unit tests.