Provide raw downloading of files in the diff viewer.
The diff viewer now shows a download icon next to the revision

information for each file, allowing the original or new versions of a

file to be downloaded. This also works with files on interdiffs.
This can be used for easily downloading a patched file at any stage of a

diff revision range, or generating a diff based on the original and

modified files outside of Review Board.
Binary files use this as well instead of the previous "Download" links.

The existing actions lists remain, but won't be shown unless there's

content on either side. That can happen if there's a "Review" or

"New Comment" link.

Compared original files and new files with their counterparts in the diff

  viewer.


Compared the files in interdiffs.


Tested a file that was modified and then had those modifications reverted.

  The downloaded files matched for both sides.


Tested the binary download links.


Unit tests passed.