make ENABLE_SHOPPING_CART truly optional

course_about was throwing 500s if ENABLE_SHOPPING_CART=False

lms/templates/courseware/course_about.html

@@ -30,29 +30,31 @@
       $("#class_enroll_form").submit();
       event.preventDefault();
     });
-    add_course_complete_handler = function(jqXHR, textStatus) {
-      if (jqXHR.status == 200) {
-        location.href = "${cart_link}";
-      }
-      if (jqXHR.status == 400) {
-        $("#register_error")
-          .html(jqXHR.responseText ? jqXHR.responseText : "${_('An error occurred. Please try again later.')}")
-          .css("display", "block");
-      }
-      else if (jqXHR.status == 403) {
-          location.href = "${reg_then_add_to_cart_link}";
-      }
-    };
-    $("#add_to_cart_post").click(function(event){
-      $.ajax({
-        url: "${reverse('add_course_to_cart', args=[course.id])}",
-        type: "POST",
-        /* Rant: HAD TO USE COMPLETE B/C PROMISE.DONE FOR SOME REASON DOES NOT WORK ON THIS PAGE. */
-        complete: add_course_complete_handler
-      })
-      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}";
+        }
+        if (jqXHR.status == 400) {
+          $("#register_error")
+            .html(jqXHR.responseText ? jqXHR.responseText : "${_('An error occurred. Please try again later.')}")
+            .css("display", "block");
+        }
+        else if (jqXHR.status == 403) {
+            location.href = "${reg_then_add_to_cart_link}";
+        }
+      };
+      $("#add_to_cart_post").click(function(event){
+        $.ajax({
+          url: "${reverse('add_course_to_cart', args=[course.id])}",
+          type: "POST",
+          /* Rant: HAD TO USE COMPLETE B/C PROMISE.DONE FOR SOME REASON DOES NOT WORK ON THIS PAGE. */
+          complete: add_course_complete_handler
+        })
+        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) {