Add usage to the dashboard

Takes the number of emails and SMS fragments sent from:
https://github.com/alphagov/notifications-api/pull/273

Using these numbers it’s possible to show:
- how much of your allowance is left
- or how much you have spent

For now the allowance and rates are hard coded.

Only for users that have manage service.

app/templates/components/big-number.html

@@ -1,7 +1,11 @@
-{% macro big_number(number, label, label_link=None) %}
+{% macro big_number(number, label, label_link=None, currency='', right_aligned=False) %}
   <div class="big-number{% if right_aligned %}-right-aligned{% endif %}">
     {% if number is number %}
-      {{ "{:,}".format(number) }}
+      {% if currency %}
+        {{ "{}{:,.2f}".format(currency, number) }}
+      {% else %}
+        {{ "{}{:,}".format(currency, number) }}
+      {% endif %}
     {% else %}
       {{ number }}
     {% endif %}