make ENABLE_SHOPPING_CART truly optional

course_about was throwing 500s if ENABLE_SHOPPING_CART=False

lms/templates/courseware/course_about.html

@@ -30,6 +30,8 @@
       $("#class_enroll_form").submit();
       event.preventDefault();
     });
+
+    % if settings.MITX_FEATURES.get('ENABLE_SHOPPING_CART') and settings.MITX_FEATURES.get('ENABLE_PAID_COURSE_REGISTRATION'):
       add_course_complete_handler = function(jqXHR, textStatus) {
         if (jqXHR.status == 200) {
           location.href = "${cart_link}";
@@ -52,7 +54,7 @@
         })
         event.preventDefault();
       });
-
+    % endif
 
     % if settings.MITX_FEATURES.get('ENABLE_SHOPPING_CART') and settings.MITX_FEATURES.get('ENABLE_PAID_COURSE_REGISTRATION'):
       add_course_complete_handler = function(jqXHR, textStatus) {