mod_actions: add create_cases_for_manual_actions option

This can also be used with a user override.
The override user is the audit log entry's author, i.e. the user who
did the kick/ban/unban.

backend/src/plugins/ModActions/ModActionsPlugin.ts

@@ -69,6 +69,7 @@ const defaultOptions = {
     can_hidecase: false,
     can_deletecase: false,
     can_act_as_other: false,
+    create_cases_for_manual_actions: true,
   },
   overrides: [
     {