Remove django.contrib.sites dependency for all components

[stevenbal]

Split off from #40 (comment)

The impact of removing sites turned out to be a bit higher than expected, see my comment here #40 (comment)

For Open Notifications and Objecttypes, it should be fairly easy to deprecate sites, since they are only used for setup_configuration. For Open Zaak and Objects API this will be more difficult, because these support notifications and create_kanaal relies on sites

• Open Zaak
• Open Notificaties
• Objects API
• Objecttypes API
• Open Klant 🔧 [maykinmedia/open-api-framework#40] Remove sites dependency open-klant#223
• Referentielijsten (sites was already uninstalled)

[alextreme]

May require an additional envvar for knowing which domain the environment is on, to be checked

Estimated: 5 days of work