fixed initialization of validation errors and flash message properties in the controller; made access denied messages more verbose regarding what action was being accessed; added support for per-usergroup homepages; handling some SQL syntax errors with exceptions - although for the future this needs to be done on the \Core\Database\Database::query() level; support for 'required' in the 'belongs to' relation specification; added support for nullable foreign keys allowing to use db compositions; changed the validation error message; removed changing the password when editing a user