Show inbound messages on the dashboard

This commit adds two things:

a section on the dashboard to show how many inbound messages the
service has received in the last 7 days, and how recently an inbound
message has been received
---

Doesn’t show the contents of any messages, just like how the rest of the
dashboard is an aggregation, never individual messages.

a page to show all the inbound messages the service has received in
the last 7 days
---

This shows the first line of the message. Eventually this will link
through to a ‘conversation’ page, where a service can see all the
messages it’s received from a given phone number.

app/main/views/dashboard.py

@@ -135,6 +135,16 @@ def monthly(service_id):
     )
 
 
+@main.route("/services/<service_id>/inbox")
+@login_required
+@user_has_permissions('manage_settings', admin_override=True)
+def inbox(service_id):
+    return render_template(
+        'views/dashboard/inbox.html',
+        messages=service_api_client.get_inbound_sms(service_id),
+    )
+
+
 def aggregate_usage(template_statistics, sort_key='count'):
     return sorted(
         template_statistics,
@@ -166,6 +176,10 @@ def get_dashboard_partials(service_id):
             'views/dashboard/_upcoming.html',
             scheduled_jobs=scheduled_jobs
         ),
+        'inbox': render_template(
+            'views/dashboard/_inbox.html',
+            inbound_sms_summary=service_api_client.get_inbound_sms_summary(service_id),
+        ),
         'totals': render_template(
             'views/dashboard/_totals.html',
             service_id=service_id,