Remove legacy multi-tenant support

Multi-tenenacy schould not live in one database.

• Turn School object into a real singleton object (using the best choice of the Django singleton modules)
• Remove SchoolRelated
• Update parent classes and unique constraints for all models in all apps and update migrations
• Make all parts that rely on the .school relation to use the School singleton

We can add multi-tenancy again in the future using one of the existing frameworks that use PostgreSQL schemas.