This change fixes the links for file attachments which are attached to

binary files in a diff. The Review UI implementation was previously just

linking to the file attachment page, but that's a pretty crummy

experience when the comments were made on the diff.
This is pretty complex because there are a lot of corner cases. In

particular, we have to deal with three annoying things:

We sometimes elide the base commit ID or tip commit ID from the URL,

   depending on what commits are selected.
When a review request has commit history, the attachment is uploaded

   for the commit-specific filediff, and then we do some magic to find

   it when the user is viewing the cumulative diff. In this case, we

   have to do the opposite and figure out if the user had commented on

   the file while viewing the cumulative diff so we can link back to

   that rather than the commit range.
When we do that backwards lookup for annoying thing #2, we need to

   make sure that we set the anchor to the correct filediff ID.

Unit tests have been added to cover all of these corner cases.

Created a review request with a ton of comments on binary files across

  various revision and commit ranges. Checked that clicking on the

  filename listed in the comments correctly opened the corresponding

  diff view.
Ran unit tests.