Adds resources for retrieving the original file, and the patched file from the Web API.

These new resources are children of the File Diff Resource, and provide a way for external tools to retrieve full copies of source files from a review request.

If the file represented by the FileDiffResource is new, the link to the OriginalFileResource will be removed. Alternatively, if the file is deleted, the link to the PatchedFileResource will be removed.

Tested manually on local dev. Ran unit tests.