Define LogEntryTypes for all actions in pretix core, improve content_object handling (#4768)

Create LogEntryType definitions for all missing action_types (order changes, check-in events, settings changes of PaymentProviders and TicketOutputs).

Check whether the stored content_object is of the expected model type, preventing incorrect links.

Refactoring:
-    Move the base LogEntryType definitions for our models to their own file
-    Move HTML escaping into make_link to make it less likely to oversee in the LogEntryType definitions
-    Log pretix.event.order.deleted with the deleted Order model as content_object, matching the other *.deleted action_types

src/pretix/base/models/orders.py

@@ -355,7 +355,7 @@ class Order(LockModel, LoggedModel):
 
         if not self.testmode:
             raise TypeError("Only test mode orders can be deleted.")
-        self.event.log_action(
+        self.log_action(
             'pretix.event.order.deleted', user=user, auth=auth,
             data={
                 'code': self.code,