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.

Unit tests pass.