R1:1049feb0ed61

Add support to Files for file storage formats, to support encryption-at-rest

Summary:
Ref T11140. When reading and writing files, we optionally apply a "storage format" to them.

The default format is "raw", which means we just store the raw data.

This change modularizes formats and adds a "rot13" format, which proves formatting works and is testable. In the future, I'll add real encryption formats.

Test Plan:
- Added unit tests.
- Viewed files in web UI.
- Changed a file's format to rot13, saw…