Set the initial scaffolding for multisite support
This commit adds the required changes for multisite support, for now only the minimum required to set up two different sites using the same database. Once this is working, we can start using things like CurrentSiteManager to look only at things that should be on this site.
To check how this work, you can use the following commands:
$ ./manage.py runserver
$ # go to http://localhost:8000/organizer/, all organizers from the SUL
$ SITE_ID=2 ./manage.py runserver
$ # Now if you go to http://localhost:8000/organizer, we only have
organizers associated with the EU instance (needs to create one in
admin)