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’

app/templates/views/choose-template.html

@@ -37,9 +37,9 @@
       {% if current_user.has_permissions(['send_texts', 'send_emails', 'send_letters'], or_=True) %}
       {{ banner(
         """
-          Send yourself a test message
+          Send yourself a test
         """,
-        subhead='Next step',
+        subhead='Next step:',
         type="tip"
       )}}
       {% endif %}