MDL-60363 conditional activities: Dim the icon also in student's view.

The class dimmed_text would only dim the activity's title and not the icon.
The teacher has both, icon and text, dimmed. So I added the class dimmed
to the class dimmed_text, what dims both elements for the students.

course/renderer.php

@@ -647,7 +647,7 @@ protected function course_section_cm_classes(cm_info $mod) {
             }
         } else {
             $linkclasses .= ' dimmed';
-            $textclasses .= ' dimmed_text';
+            $textclasses .= ' dimmed dimmed_text';
         }
         return array($linkclasses, $textclasses);
     }