Update RB.DiffReviewableView to modern standards

Review Request #10194 — Created Oct. 4, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
0d3a1df...

Reviewers

This patch updates RB.DiffReviewableView and its unit tests to use
ES6 features and our documentation standards. Although the
RB.DiffReviewableView was written in an ES6 file, it was still using
var everywhere over let and const. It was also wrapped in a
superfluous IIFE.

Ran JS tests.