The file attachment UI previously did not have any way to navigate

between any file attachments. A user would have to navigate back to the

review request and select the next one they wished to review.
Now there are previous and next buttons on the edges of the page when

there are more file attachments to review. They follow the order of the

review request UI and only navigate to active (and latest revisions).

When they are hovered over, they expand out to show the thumbnail of the

attachment that will be navigated to.

Ran unit tests.
Tested in the following browsers on OSX:

Chrome
Firefox release
Firefox developer edition
Safari

Verified the links were correct on local site review requests.