Removes hard coded links to /admin

The _editor_js.html page has been modified in order to get the urls to the various views through the {% url %} tag and add them to the global window variable. I believe that this is actually the best solution in our case and I like it much better than the other available options because the other options were either to add a new view that would return a javascript array with the urls or to add a management command that would generate a static javascript file with the same array. In both cases we'd need to add *more* instructions to the installation of wagtail and confuse people wanting to use it more. In this solution, the links just work :)