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.

tests/app/test_utils.py

@@ -156,5 +156,7 @@ def test_generate_notifications_csv_calls_twice_if_next_link(mocker):
     assert mock_get_notifications.mock_calls[1][2]['page'] == 2
 
 
-def normalize_spaces(string):
-    return ' '.join(string.split())
+def normalize_spaces(input):
+    if isinstance(input, str):
+        return ' '.join(input.split())
+    return normalize_spaces(' '.join(item.text for item in input))