When we render binary file review UIs in the diff viewer, we receive an

HTML fragment from the server which we inject into the diff viewer. That

HTML includes JavaScript which creates the reviewable view and model objects

for the file.
When we change the revision or commits in the diff viewer, the HTML gets

removed from the DOM. However, any Backbone or other JS objects that were

created in that HTML fragment don’t actually get destroyed or cleaned up.

They end up holding unnecessary memory in the browser.
To address this, whenever we clear the diff viewer we emit a signal that

FileAttachmentReviewableViews will listen to and remove themselves.

Subclasses can override onRemove() to perform any additional cleanup

needed for their review UI.

Ran JS unit tests.
Used in a change where we clean up PDF and Office document review

  UIs in the diff viewer.
Tested changing the revision of a diff viewer that contains binary

  image files.