Add silos, which will isolate organizations, and provide a namespace for
users and groups.

This required adding Silo id to Actor, so users that have authenticated
now have an associated Silo id that can be used to restrict organization
lookup.

Silos can be created, read, and deleted. Modification is a TODO. Silos
can be marked discoverable or not, to support a tenancy model where
users and resources are strongly isolated from each other.

A few tests have been modified to use authn_as because an earlier
version of this branch added OpContext to every endpoint, but that was
reverted because the blast radius of the PR would have been too large.
What remains are a few modified tests that make authenticated calls.

When all endpoints are protected and each datastore function has an
OpContext, Silo can be looked up on Actor. For now, there are places
hard coding as the built-in Silo.

Still TODO:
- authz for silos and silo users
  - some testing is dependent on ^
- PUT /silos/{name}
- building on top of silos