Integrate django-scopes (#1319)

* Install django-scopes

* Fix tests.api

* Update tasks and cronjobs

* Fix remaining tests

* Remove unused import

* Fix tests after rebase

* Disable scopes for get_Events_with_any_permission

* Disable scopes for a management command

src/pretix/testutils/scope.py

@@ -0,0 +1,10 @@
+from django_scopes import scope
+
+
+def classscope(attr='o'):
+    def wrap(fn):
+        def wrapped(self, *args, **kwargs):
+            with scope(organizer=getattr(self, attr)):
+                return fn(self, *args, **kwargs)
+        return wrapped
+    return wrap