Fix fetching original and patched files through the web API.

The handlers for getting original/patched files in the API were
expecting a URL parameter that doesn't exist. It would always attempt to
fetch using the draft filediff resource, instead of intelligently
choosing between that and the published filediff resource.

By using the correct URL parameter name, and using that parameter
correctly, we can fetch the original/patched files correctly.

Reviewed at https://reviews.reviewboard.org/r/5804/