Mail links

* adds direct object links into mail templates
* refactors transferring app-model identification data from fore- to background (celery) properly

templates/email/other/deduction_changed.html

@@ -10,6 +10,7 @@
         <br>
         {% translate 'a deduction of this eco account has changed:' %}
         <br>
+        <a href="{{ obj.get_detail_url_absolute }}"><strong>{{obj.identifier}}</strong></a>
         <br>
         <table>
             <tr>