R1:bb4904553f2c

Route some VCS connections over SSH

Summary:
- Add web UI for configuring SSH hosting.
- Route git reads (`git-upload-pack` over SSH).

Test Plan:
>>> orbital ~ $ git clone ssh://127.0.0.1/
Cloning into '127.0.0.1'...
Exception: Unrecognized repository path "/". Expected a path like "/diffusion/X/".
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
>>> orbital ~ $ git clone ssh://127.0.0.1/diffusion/X/
Cloning…