MDL-71189 core_completion: Adding completion sort order to activities

This implements sort ordering for all completion conditions, in
activities which support custom completion conditions.

mod/feedback/classes/completion/custom_completion.php

@@ -67,4 +67,16 @@ class custom_completion extends activity_custom_completion {
             'completionsubmit' => get_string('completiondetail:submit', 'feedback')
         ];
     }
+
+    /**
+     * Returns an array of all completion rules, in the order they should be displayed to users.
+     *
+     * @return array
+     */
+    public function get_sort_order(): array {
+        return [
+            'completionview',
+            'completionsubmit',
+        ];
+    }
 }