RB.DiffViewerPageView has been responsible for a lot of state and

logic around loading in diffs from the server. This change moves some of

that down to the RB.DiffViewerPage model. The model now handles

creating/storing all RB.DiffReviewables and emitting events that the

view can use to know when to queue up the loading of diffs or when to

reset elements for a new set.
By moving state to the model, it'll be easier for the new review UI work

and the diff bookmarks features to better track and respond to updates

made on the page.

All unit tests pass.
Tested viewing several diffs, changing pages, and viewing different

revisions/interdiffs.
Tested navigating via keybindings and the file list.
Tested commenting on lines.
Tested expanding/collapsing context in a diff.