The built-in Silo with its built-in users have been useful, but they were always intended as a stepping stone until we had more real facilities. Once we do #2304, I think we'll be able to remove these.

The remaining hard part is probably the test suite. We may want to have it create its own Silo with these same test users. The difference will only be that these are created by the test suite rather than automatically by Nexus. This is kind of a big difference in terms of security, since these test users won't be present in real systems.

Depends on #2304.