fix: Prevent datetimeshortcuts from rendering multiple times (#129)

This was happening because scripts injected inside
`$(document).ready(...)` would delay onload event and django's
DateTimeShortcuts script attaches an event handler to `load` event to
initialize the widgets. What would happen is our own initialization
would run first and the one coming from django second resulting in
duplicate widget. This change ensures our initialization happens last.

See #128

rangefilter/templates/rangefilter/date_filter.html

@@ -131,7 +131,7 @@
         });
     }
 
-    django.jQuery('document').ready(function () {
+    django.jQuery(window).on('load', function () {
         if (!('DateTimeShortcuts' in window)) {
             var promiseList = [];
 

rangefilter/templates/rangefilter/date_filter_4_0.html

@@ -127,7 +127,7 @@ <h3>{{ title }}</h3>
         });
     }
 
-    django.jQuery('document').ready(function () {
+    django.jQuery(window).on('load', function () {
         if (!('DateTimeShortcuts' in window)) {
             var promiseList = [];