R1:0a4cbdff5e59

Straighten out Diffusion file integration

Summary:
This is in preparation for getting the "View Options" dropdown working on audits.

- Use Files to serve raw data so we get all the security benefits of the alternate file domain. Although the difficulty of exploiting this is high (you need commit access to the repo) there's no reason to leave it dangling.
- Add a "contentHash" to Files so we can lookup files by content rather than adding some weird linker table. We can do other things with this later…