MDL-71189 core_completion: Implementing custom completion sort ordering

This adds the requirement for activities supporting custom completion to
specify the order all completion conditions should be displayed for that
activity. It also implements the sorting that takes place.

lib/upgrade.txt

@@ -109,6 +109,9 @@ information provided here is intended especially for developers.
   - get_custom_rule_descriptions(): Returns an associative array with values containing the user-facing textual description
     of the custom completion rules (which serve as the keys to these values).
     e.g. ['completionsubmit' => 'Must submit']
+  - get_sort_order(): Returns an array listing the order the activity module's completion rules should be displayed to the user,
+    including both custom completion and relevant core completion rules
+    e.g. ['completionview', 'completionsubmit', 'completionusegrade']
 * Admin setting admin_setting_configmulticheckbox now supports lazy-loading the options list by
   supplying a callback function instead of an array of options.
 * A new core API class \core_user\fields provides ways to get lists of user fields, and SQL related to