The "Download Diff" action was hardcoding the URL for the revision of

the review request on first load. However, it was not updating the link

when the revision changed. Therefore, this action has been updated to

be a relative link to raw/ in the diffviewer, which was what the

previous code did. This way, it will always link to the correct diff

revision. We still use an absolute URL on the review request page

itself to generate a correct link.
In addition, that action would be visible on review requests without

diffs due to a logic error (comparing the empty list to None). We now

just check if the review request has a repository instead.
There have also been some edits made to the action template to generate

HTML that looks a bit better when rendered.
Unit tests have been updated to address these issues. Unit tests for the

download diff action's URL have been removed becuase they are no longer

necessary.

Ran unit tests.
Verified the "Download Diff" action links to the currently selected

  diff revision.