In the diff viewer, it's often useful to evaluate whether or not it was

right to remove a file. Currently, we instead see the message:
This file was deleted. The content cannot be displayed.

This change will make it possible to view deleted file content by

providing a clickable link that is labelled "Show content.". An event

listener will requeue the corresponding diff file, where eventually a

GET request ending in the following will be made:
fragment/<FileDiffID>/?<OtherQueryStringPairs>&show-deleted=1

To accomplish this without running into any potential caching issues, we

added the show_deleted flag to the ETag of the corresponding diff file.

Ran and passed all relevant reviewboard unit tests.
Manually verified that deleted files could be correctly displayed on

  Chrome and Firefox without triggering a full page reload, and that

  diff files were cached correctly.