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 call, the client pulls the file data separately.

We also no longer need to return a complex data structure because we don't do blame over this call any longer.

Test Plan:

• Viewed images in Diffusion.
• Viewed READMEs in Diffusion.
• Used bin/differential attach-commit rX Dy to hit attach pathway.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9319

Differential Revision: https://secure.phabricator.com/D14970