Add a page to show delivery rates week-by-week

Implements
https://github.com/alphagov/notifications-api/pull/286

Will always show weeks as Monday to Sunday.

app/templates/views/dashboard/dashboard.html

@@ -20,6 +20,9 @@
       {% include 'views/dashboard/no-permissions-banner.html' %}
     {% endif %}
 
+    <h1 class="heading-medium">
+      In the last 7 days
+    </h1>
     {% include 'views/dashboard/today.html' %}
 
   </div>