Show template usage broken down by month

This follows on from:
- https://github.com/alphagov/notifications-admin/pull/1094
- https://github.com/alphagov/notifications-admin/pull/1109

It depends on:
- [ ] https://github.com/alphagov/notifications-api/pull/829

A year is too long. Month-by-month is a better timeframe for making
decisions or seeing patterns in your usage.

app/assets/stylesheets/views/dashboard.scss

@@ -81,3 +81,16 @@
   @include bold-19;
   color: $error-colour;
 }
+
+.template-usage-table {
+
+  border-top: 1px solid $border-colour;
+  border-bottom: 1px solid $border-colour;
+  margin-top: 10px;
+  margin-bottom: $gutter * 1.3333;
+
+  .table {
+    margin-bottom: 5px;
+  }
+
+}