Move the diff revision selector into javascript.

Rather than generating this from the django template and including links, it's
now generated on the client-side, and clicks are handled by triggering the
"revisionSelected" event. This still ends up navigating to another page, but
it's a step towards avoiding the reload.

Testing done:
* Navigated to different diff and interdiff revisions.
* Ran js-tests.
* Ran jshint.

Reviewed at https://reviews.reviewboard.org/r/4565/