Wire in the new API method that calculates the total cost and total billable units.

- Used on dashboard to calculate free tier/cost
- Update tests to mock new method
- Two new tests to check output on dashboard page

app/templates/views/dashboard/_usage.html

@@ -10,7 +10,7 @@
     <div class="keyline-block">
       {% if sms_chargeable %}
         {{ big_number(
-          (sms_chargeable * sms_rate),
+          total_sms_cost,
           'spent on text messages',
           currency="£",
           smaller=True