Mail links

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

templates/email/sharing/shared_access_removed.html

@@ -10,7 +10,7 @@
         <br>
         {% trans 'your shared access, including editing, has been revoked for the dataset ' %}
         <br>
-        <strong>{{obj.identifier}}</strong>
+        <a href="{{ obj.get_detail_url_absolute }}"><strong>{{obj.identifier}}</strong></a>
         <br>
         <strong>"{{obj.title}}"</strong>
         <br>