Support variable rates for annual usage stats

Note: I've removed the pricing assertion in the "0_free_allowance"
test as it's covered elsewhere - the value of the test is really to
check that we don't show the remainder if there never was any.

app/main/views/dashboard.py

@@ -328,8 +328,6 @@ def get_dashboard_totals(statistics):
 
 def get_annual_usage_breakdown(usage, free_sms_fragment_limit):
     sms = get_usage_breakdown_by_type(usage, 'sms')
-    # this relies on the assumption: only one SMS rate per financial year.
-    sms_rate = 0 if len(sms) == 0 else sms[0].get("rate", 0)
     sms_chargeable_units = sum(row['chargeable_units'] for row in sms)
     sms_free_allowance = free_sms_fragment_limit
     sms_cost = sum(row['cost'] for row in sms)
@@ -346,9 +344,8 @@ def get_annual_usage_breakdown(usage, free_sms_fragment_limit):
         'sms_free_allowance': sms_free_allowance,
         'sms_sent': sms_chargeable_units,
         'sms_allowance_remaining': max(0, (sms_free_allowance - sms_chargeable_units)),
-        'sms_charged': max(0, sms_chargeable_units - sms_free_allowance),
         'sms_cost': sms_cost,
-        'sms_rate': sms_rate,
+        'sms_breakdown': sms,
         'letter_sent': letters_sent,
         'letter_cost': letters_cost
     }

app/templates/views/usage.html

@@ -33,13 +33,15 @@
             {% if sms_free_allowance > 0 %}
               {{ big_number(sms_allowance_remaining, 'free allowance remaining', smaller=True) }}
             {% endif %}
-            {% if sms_charged %}
-              {{ big_number(
-                sms_charged,
-                'at {:.2f} pence per message'.format(sms_rate * 100),
-                smaller=True
-              ) }}
-            {% endif %}
+            {% for row in sms_breakdown %}
+              {% if row.charged_units > 0 %}
+                {{ big_number(
+                  row.charged_units,
+                  'at {:.2f} pence per message'.format(row.rate * 100),
+                  smaller=True
+                ) }}
+              {% endif %}
+            {% endfor %}
           </div>
         </div>
         <div class='govuk-grid-column-one-third'>