Fix the display of binary files in the diff viewer.

We had some broken logic that excluded binary files from the diff viewer.
It also may have added some stray </table> tags in some cases.

This change updates our diff_file_fragment.html template to account for
binary files, to display them properly, and to restructure some of the
logic to be more maintainable and readable and a little less confusing.

Tested this with standard diffs, interdiffs, interdiffs where a file didn't change and was removed, and with binary files.