R1:d1fb2f7fb96d

Make `diffusion.filecontentquery` return file PHIDs instead of raw content

Summary:
Fixes T9319. Proxied requests (e.g., in the cluster) for binary files (like images) currently fail because we can not return binary data over Conduit in JSON.

Although Conduit will eventually support binary-safe encodings, a cleaner approach to this is just to return a `filePHID` instead of the raw content. This is generally faster and more flexible, and gives us more opportunities to add caching later.

After making the…