Always draw images with Javascript

Summary:
Currently, we use two different methods to draw Mock main images:

  - When the page first loads, we draw the image on the server with PHP.
  - When the user clicks a thumbnail, we draw the image on the client with JS.

Since we can't reasonably get rid of the client version of this, move it all to the client. This paves the way for some future features, like:

  - Doing some magic with stage sizing
  - URIs that jump to a specific comment
  - showing "loading" indicators while images are loading
  - Loading lowsrc images first?

Test Plan: Loaded page, clicked thumbs.

Reviewers: ljalonen, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5073