Add monthly breakdown of usage

Basically:
- shows all the months from start of given financial year to now or end
  of given financial year (whichever is earliest)
- shows a breakdown of free and paid text messages for each of these
  months

Depends on:
- [x] https://github.com/alphagov/notifications-api/pull/699

app/assets/stylesheets/components/table.scss

@@ -31,7 +31,7 @@
   }
 
   .table-field-heading-first {
-    width: 52.5%
+    width: 52.5%;
   }
 
   .table-row {