This change introduces a handful of new JavaScript models for

representing data going into the review request entry boxes (change

descriptions, initial status updates, and reviews). These store the

ReviewRequestEditor used, data on review(s) (for the review box and

status updates), and data on the diff comments (used to load diff

fragments).
The templates for these entries no longer need to call JavaScript

methods, beyond adding the boxes and models. The data is all

self-contained. This will later allow for more easily reloading data for

entries without reloading the entire page.
The new models live in a js/reviewRequestPage/ directory. We'll be able

to move other models/views specific to the review request page here

later on, helping keep things organized.

Unit tests passed.
Tested loading diff comments for reviews (in all boxes) and replying to

them.