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/
Clean up the view_diff.html template.

This change removes some obsolete variable definitions, fixes indentation on
template tags, and generally cleans things up.

Testing done:
Played around with the diff viewer a bit and checked that things looked right.

Reviewed at https://reviews.reviewboard.org/r/4568/
Provide WebAPIResource in reviewboard.webapi.resources again.

The recent API codebase restructuring moved WebAPIResource into
reviewboard.webapi.base, but older extensions expected this to be found
in reviewboard.webapi.resources. That's completely valid, so now we
provide it there as well.

Older extensions importing the various resource instances will still
need to be updated, though.