Clean up comment block and reviewable models

Review Request #10191 — Created Oct. 3, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
787f277...

Reviewers

The RB.AbstractCommentBlock and RB.AbstractReviewable heirarchies
needed some cleaning up. Most of them were lacking documentation about
what their model attributes were. Also the RB.DiffCommentBlock model
was using ES5 style method: function() {...} definitions instead of
ES6 style method() {...} definitions. Finally, the
RB.FileAttachmentReviewable model was re-defining the caption
attribute which it inherits from its parent model.

All of these issues have been cleaned up. All model attributes are
documented with links back to the parent models for additional
attributes.

Ran JS tests.