Convert "Reviewable" model and view classes to ES6.

Review Request #8556 — Created Nov. 29, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
6188e66...

Reviewers

This is a pretty straightforward transformation of all the remaining
reviewables. The transformations in here are pretty straightforward: fixing up
documentation comments, making use of some additional syntactic sugar,
switching var for let/const, etc.

Ran js-tests.