Mail links

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

templates/email/resubmission/resubmission.html

@@ -2,7 +2,7 @@
 
 <div>
     <h2>{% trans 'Resubmission' %}</h2>
-    <h4>{{obj.identifier}}</h4>
+    <a href="{{ obj.get_detail_url_absolute }}"><h4>{{obj.identifier}}</h4></a>
     <hr>
     <article>
         {% trans 'Hello ' %} {{resubmission.user.username}},