Wrap js in DOMContentLoaded listener to prevent $.ui from being undefined

[ErwinJunge]

I had a problem after upgrading to django 2.2 that $.ui was undefined on my admin pages and selectable stopped working, even though both jquery and jqueryui were still in the head (we use the base_site.html override with include_ui_theme and include_jquery_libs).

It's a bit hard to see from the diff, but essentially the fix comes down to wrapping the js in

document.addEventListener("DOMContentLoaded", function() {
   // js goes here
});