Return to preserved filters on change_list after object action (#88)

If you want to filter change_list by some conditions,
then open each object and run some object action there,
you will be returned to change_list with the same preserved filters.

Fix #83 

* Return to preserved filters on change_list after object action

* Changes after pull request: Load add_preserved_filters from admin_urls

* Changes after pull request: Indentation presumably reverted to original style

django_object_actions/templates/django_object_actions/change_form.html

@@ -1,10 +1,11 @@
 {% extends "admin/change_form.html" %}
-
+{% load add_preserved_filters from admin_urls %}
 
 {% block object-tools-items %}
   {% for tool in objectactions %}
     <li class="objectaction-item" data-tool-name="{{ tool.name }}">
-      <a href='{% url tools_view_name pk=object_id tool=tool.name %}' title="{{ tool.standard_attrs.title }}"
+      {% url tools_view_name pk=object_id tool=tool.name as action_url %}
+      <a href="{% add_preserved_filters action_url %}" title="{{ tool.standard_attrs.title }}"
          {% for k, v in tool.custom_attrs.items %}
            {{ k }}="{{ v }}"
          {% endfor %}

django_object_actions/templates/django_object_actions/change_list.html

@@ -1,10 +1,11 @@
 {% extends "admin/change_list.html" %}
-
+{% load add_preserved_filters from admin_urls %}
 
 {% block object-tools-items %}
   {% for tool in objectactions %}
     <li class="objectaction-item" data-tool-name="{{ tool.name }}">
-      <a href='{% url tools_view_name tool=tool.name %}' title="{{ tool.standard_attrs.title }}"
+      {% url tools_view_name pk=object_id tool=tool.name as action_url %}
+      <a href="{% add_preserved_filters action_url %}" title="{{ tool.standard_attrs.title }}"
          {% for k, v in tool.custom_attrs.items %}
            {{ k }}="{{ v }}"
          {% endfor %}