Make date of sending relative

This is a friendlier and better way of showing dates anywhere except in
a report which might be archived and referred back to later.

Bit trickier to implement here because a dat requires ‘on’ beforehand,
but we don’t say ‘on today’ in English.

app/templates/views/notifications/notification.html

@@ -34,7 +34,7 @@
       {% elif created_by %}
         by {{ created_by.name }}
       {% endif %}
-      on {{ created_at|format_datetime_short }}
+      {{ created_at|format_datetime_human }}
     </p>
 
     {% if template.template_type == 'letter' %}