Put usage summary back on the dashboard

This reverts 1b1839ad30, which removed
the usage from the dashboard because it was causing performance
problems:

> **The yearly usage section on the dashboard page takes too log as a
> result services with large yearly stats are timing out.**
>
> As a short term fix we have taken the yearly stats off the dashboard.
>
> There is a plan to create permanent statistic tables to warehouse the
> data.

The long term fix (the fact tables) is now in place, so it should be OK
to bring this back.

This is part of a wider piece of work to refresh the dashboard page now
that jobs are moving to their own page.

app/templates/views/dashboard/_usage.html

@@ -1,16 +1,16 @@
 {% from "components/big-number.html" import big_number %}
 
 <div class='grid-row ajax-block-container'>
-  <div class='column-half'>
+  <div class='{{ column_width }}'>
     <div class="keyline-block">
       {{ big_number("Unlimited", 'free email allowance', smaller=True) }}
     </div>
   </div>
-  <div class='column-half'>
+  <div class='{{ column_width }}'>
     <div class="keyline-block">
       {% if sms_chargeable %}
         {{ big_number(
-          total_sms_cost,
+          sms_chargeable * sms_rate,
           'spent on text messages',
           currency="£",
           smaller=True
@@ -20,4 +20,14 @@
       {% endif %}
     </div>
   </div>
+  <div class='{{ column_width }}'>
+    <div class="keyline-block">
+      {{ big_number(
+        letter_cost,
+        'spent on letters',
+        currency="£",
+        smaller=True
+      ) }}
+    </div>
+  </div>
 </div>