Simple virtual hosting idea

It occurs to me that collectives can be (ab)used to do virtual hosting. The following steps would be needed:

1. include a table in the database and/or config to map HTTP Hosts to collectives;
2. for each domain/collective, allow for the specification of its own SSO configuration, including the base URL;
3. for each domain/collective, load a custom stylesheet (#2413);

With these in place, it seems trivial to serve two domains/collectives with the same restserver/joex pair.