Mail sending

* adds mail sending logic for new notification setting
* adds new templates for user and team based sending
* enhances all email template layout
* adds translations

templates/email/deleting/shared_data_deleted.html

@@ -7,6 +7,7 @@
     <article>
         {% trans 'Hello ' %} {{user.username}},
         <br>
+        <br>
         {% trans 'the following dataset has just been deleted' %}
         <br>
         <strong>{{obj_identifier}}</strong>