The RB.DiffViewerPage model had a few helper child objects that were

responsible for different aspects of the page. These were stored as

attributes, in order to allow usage of parse() to feed in data. That

required also modifying set() to be able to update the existing

instances based on the attributes of newly-constructed ones from

parse(), and made working with the objects a bit awkward.
This change switches these objects to being constructed when creating

an instance of the model, and then filling the objects in parse(). We

use Model.constructor() instead of Model.initialize() to do this,

since the former will run before parse() and initialize(), allowing

us to create those objects. This is recommended in the Backbone

documentation for this use case.

Unit tests pass.
Tested loading diffs, switching revisions (single revisions and

interdiff ranges), and switching pages.