Enterprise feature: Ability for administrators to disable SSH for groups or site-wide

A Coder prospect is a financial institution and wants the ability to:

1. Disable SSH (at a site or group-level)
2. If they allow SSH (for local IDE support), they want to prevent exfiltration, e.g., `scp` `rsync` `mutagen`

2 is the optimal state but 1 is Coder v1 functionality so should be ported for parity.