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.