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/assets/stylesheets/components/big-number.scss

@@ -11,6 +11,12 @@
 
 }
 
+.big-number-right-aligned {
+  @extend %big-number;
+  @include bold-36($tabular-numbers: true);
+  text-align: right;
+}
+
 .big-number-with-status {
 
   @extend %big-number;