Addons: resolve URLs for file tree diff API response (#11749)
Resolve all the filename into URLs, so the frontend can use these links
without manipulating them. We don't want re-implement the resolver in
the frontend.
Now, we return the filename, the URL for the current version, and the
URL for the source version of the filename changed.
Closes https://github.com/readthedocs/addons/issues/409