Put some statistics on the dashboard

This commit adds two new sections to the dashboard

1. A banner telling you about trial mode, including a count of how many messages
   you have left today, which is a restriction of trial mode

2. Panels with counts of how many emails and text messages have been sent in a
   day, plus the failure rates for each

It does **not**:
- link through to any further information about what trial mode is (coming
  later)
- link through to pages for the failure rates (coming later)
- change the ‘recent jobs’ section to ‘recent notifications’

tests/app/main/views/test_accept_invite.py

@@ -275,6 +275,7 @@ def test_new_invited_user_verifies_and_added_to_service(app_,
                                                         mock_accept_invite,
                                                         mock_get_service,
                                                         mock_get_service_templates,
+                                                        mock_get_service_statistics,
                                                         mock_get_jobs):
 
     with app_.test_request_context():