MDL-37030 Assignment: Fix error on download all submissions.

When group submissions and blind marking are enabled, an error is thrown
when a teacher trys to download all submissions in a zip. The fix is to
move the url rewriting from the plugin to the assign class so it is
done in a standard way by all modules. The rewriting is done to make images
in a text editor field resolve correctly.

mod/assign/assignmentplugin.php

@@ -415,9 +415,11 @@ abstract class assign_plugin {
      *
      * @param stdClass $submissionorgrade assign_submission or assign_grade
      *                 For submission plugins this is the submission data, for feedback plugins it is the grade data
+     * @param stdClass $user The user record for the current submission.
+     *                         Needed for url rewriting if this is a group submission.
      * @return array - return an array of files indexed by filename
      */
-    public function get_files(stdClass $submissionorgrade) {
+    public function get_files(stdClass $submissionorgrade, stdClass $user) {
         return array();
     }