MDL-37710 Assign: Fix pluginfile permission checks for student viewing their own team submission.

This change adds a function to the assign class to allow the permissions for a group submission
to be checked and updates all the submission plugins to call it.

mod/assign/locallib.php

@@ -2784,6 +2784,30 @@ class assign {
         return false;
     }
 
+    /**
+     * Perform an access check to see if the current $USER can view this group submission.
+     *
+     * @param int $groupid
+     * @return bool
+     */
+    public function can_view_group_submission($groupid) {
+        global $USER;
+
+        if (!is_enrolled($this->get_course_context(), $USER->id)) {
+            return false;
+        }
+        if (has_capability('mod/assign:grade', $this->context)) {
+            return true;
+        }
+        $members = $this->get_submission_group_members($groupid, true);
+        foreach ($members as $member) {
+            if ($member->id == $USER->id) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     /**
      * Perform an access check to see if the current $USER can view this users submission.
      *