Improve JS behavior when editing slug (fixes #132)

wagtail/wagtailadmin/static/wagtailadmin/js/page-editor.js

@@ -261,8 +261,20 @@ function initSlugAutoPopulate(){
 }
 
 function initSlugCleaning(){
-    $('#id_slug').on('keyup blur', function(){
-        $(this).val(cleanForSlug($(this).val()));
+    $('#id_slug').on({
+        'blur': function(){
+            $(this).val(cleanForSlug($(this).val()));
+        },
+        'keyup': function(event){        
+            var keyCode = ('which' in event) ? event.which : event.keyCode;
+            if (
+                (keyCode != 32) && // not spacebar
+                (keyCode != 8) &&  // not backspace
+                (keyCode != 46) && // not delete
+                ((keyCode < 40) ) // non-character
+            ) return ;
+            $(this).val(cleanForSlug($(this).val()));
+        }
     });
 }