Use per-service data retention period to display notifications page

Uses the configured service data retention page to display retention
period length, notification counts and fetch notifications from the
API on the notifications page.

app/templates/views/activity/notifications.html

@@ -11,7 +11,7 @@
       notifications,
       caption="Recent activity",
       caption_visible=False,
-      empty_message='No messages found  (messages are kept for 7 days)'|safe,
+      empty_message='No messages found  (messages are kept for {} days)'.format(limit_days)|safe,
       field_headings=['Recipient', 'Status'],
       field_headings_visible=False
     ) %}