mmg rates set to 1.65p

also moved a default parameter to a plain ol' local variable to reduce ambiguity about whether it is overriden by any callers (it's not)

app/templates/views/usage.html

@@ -34,7 +34,7 @@
           {% if sms_chargeable %}
             {{ big_number(
               sms_chargeable,
-              'at {:.1f}p per message'.format(sms_rate * 100),
+              'at {:.2f}p per message'.format(sms_rate * 100),
               smaller=True
             ) }}
           {% endif %}