Update the draft filediff test suite to use the new mixins.

The draft filediff support was written before all the new test mixins,
but committed after merging those mixins. It was never updated to use
them. This change fixes that.
Add API for fetching and setting diff file attachments.

There's now a /api/repositories/<id>/diff-file-attachments/ resource
that contains the known, registered diff file attachments tied to a
repository. Nothing on this tree can be modified, only retrieved.

In the future, some file attachments will be automatically associated.
Those that cannot can be modified by issuing a PUT on a draft filediff
resource and passing a file for the file attachment representing the
destination/modified file. Right now, this is allowed for all filediffs
for binary files, but later it will be restricted.