R1:476b27d9c8b2

Add "phd.user" with `sudo` hooks for SSH/HTTP writes

Summary:
Ref T2230. When fully set up, we have up to three users who all need to write into the repositories:

- The webserver needs to write for HTTP receives.
- The SSH user needs to write for SSH receives.
- The daemons need to write for "git fetch", "git clone", etc.

These three users don't need to be different, but in practice they are often not likely to all be the same user. If for no other reason, making them all the same user requires…