fix: Facebook share button on course certificates.

Facebook requires a callback when using the `.ui()` method now. I don't
know when this changed, but the share button is currently broken on
course certificates.

Facebook documentation of the `.ui()` method:
https://developers.facebook.com/docs/javascript/reference/FB.ui/

lms/static/js/utils/facebook.js

@@ -21,13 +21,18 @@ var FaceBook = (function() {
             }(document, 'script', 'facebook-jssdk'));
         },
         share: function(feed_data) {
-            FB.ui({
-                method: 'feed',
-                name: feed_data.share_text,
-                link: feed_data.share_link,
-                picture: feed_data.picture_link,
-                description: feed_data.description
-            });
+            FB.ui( // eslint-disable-line no-undef
+                {
+                    method: 'feed',
+                    name: feed_data.share_text,
+                    link: feed_data.share_link,
+                    picture: feed_data.picture_link,
+                    description: feed_data.description
+                },
+                // The Facebook API now requires a callback. Since we weren't doing anything after posting before,
+                // I'm leaving this as an empty function.
+                function(response) {} // eslint-disable-line no-unused-vars
+            );
         }
     };
 }());