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/assets/stylesheets/_grids.scss

@@ -2,6 +2,10 @@
   @include grid-column(3/4);
 }
 
+.column-one-sixth {
+  @include grid-column(1/6);
+}
+
 .column-one-eighth {
   @include grid-column(1/8);
 }
@@ -16,7 +20,7 @@
 }
 
 .bottom-gutter-2-3 {
-  margin-bottom: $gutter * 2/3;
+  margin-bottom: $gutter-two-thirds;
 }
 
 .align-with-heading {