Add a set of views for managing reviewable pages.

This introduces three new views for managing the reviewable pages.

ReviewablePageView is the main/base class for reviewable pages. It
constructs all the important objects, listens for updates on the review
request from the server, and handles certain page actions.

ReviewRequestPageView extends that, and owns the ReviewBoxListView and
the diff fragment loading.

DiffViewerPageView is for the diff viewer page. It's very small right
now, but will grow.

These are instantiated on the appropriate pages. The bulk of the
initialization comes from a .js template file, which handles all the
main parameters to the page views. The pages can provide additional
parameters as well.

With this, I was able to get rid of a bunch more globals, which is
always good.

Also, reviews.js is now gone! Tada!

Tested that each page loaded its contents correctly.

Tested update notification bubbles.

Tested that reviewing code and images works.

Tested the review box expand/collapse functionality.

Tested clicking "Reply" in the comment dialog for an existing comment
opened the review request page and opened a comment box for the correct
comment.

Unit tests pass.