We had `getgrist.com` hardcoded here, which only works for SaaS. The
base domain as well as the way that orgs are encoded in the URL can be
different in other circumstances.
If we are encoding orgs in the domain name, that's easy. We just do
`orgname.base.domain.name`. If we are not, then we first try a base
domain, and if that isn't set, we'll use the domain of the home
server.