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/assets/stylesheets/views/dashboard.scss

@@ -94,3 +94,7 @@
   }
 
 }
+
+.align-with-message-body {
+  margin-top: $gutter * 5 / 6;
+}