Assign isExternal JS function to window object

When JS functions are defined with names, they are local variables, and inaccessible
if defined inside a closure. Django-Pipeline concatenates all of our JS into one
big closure. This function explicitly assings the function to a property of the
`window` object, so that it is accessible to other JS functions.

common/static/js/utility.js

 // checks whether or not the url is external to the local site.
 // generously provided by StackOverflow: http://stackoverflow.com/questions/6238351/fastest-way-to-detect-external-urls
-function isExternal(url) {
+window.isExternal = function (url) {
     // parse the url into protocol, host, path, query, and fragment. More information can be found here: http://tools.ietf.org/html/rfc3986#appendix-B
     var match = url.match(/^([^:\/?#]+:)?(?:\/\/([^\/?#]*))?([^?#]+)?(\?[^#]*)?(#.*)?/);
     // match[1] matches a protocol if one exists in the url
     // if the protocol in the url does not match the protocol in the window's location, this url is considered external
-    if (typeof match[1] === "string" && 
-            match[1].length > 0 
-            && match[1].toLowerCase() !== location.protocol) 
+    if (typeof match[1] === "string" &&
+            match[1].length > 0 &&
+            match[1].toLowerCase() !== location.protocol)
         return true;
     // match[2] matches the host if one exists in the url
     // if the host in the url does not match the host of the window location, this url is considered external
-    if (typeof match[2] === "string" && 
-            match[2].length > 0 && 
+    if (typeof match[2] === "string" &&
+            match[2].length > 0 &&
             // this regex removes the port number if it patches the current location's protocol
-            match[2].replace(new RegExp(":("+{"http:":80,"https:":443}[location.protocol]+")?$"), "") !== location.host) 
+            match[2].replace(new RegExp(":("+{"http:":80,"https:":443}[location.protocol]+")?$"), "") !== location.host)
         return true;
     return false;
-}
+};