Add rendering of binary file attachments in the diff viewer.

This introduces the ability to associate a FileAttachment with a
path, revision and repository. The diff viewer can make use of this to
render thumbnails for binary files.

There's no way yet of associating a file outside of modifying the
database. Later, there will be API and UI for this.

This work is based on Tina Yang's work, with some fixes and a change in
the association model. Previously, FileAttachments were associated with
FileDiffs, but have been changed in order to reduce the number of
duplicate files that exist in the database.

Generated a diff that introduced an image and modified another image.

Initially, I saw the standard "This is a binary file," except with the
Download links and the text in both the left-hand and right-hand sides.

I then manually associated file attachments with these files. They didn't
show up in the list of file attachments for the page, but they did show up
in the diff viewer in the appropriate places.