Reword and embiggen the headings

Also reword some of the `<p>`s so that they flow better from the
headings.

This was a bit me, and a bit sitting down and hammering this stuff out
with Stephen and Sheryll.

app/assets/stylesheets/views/product-page.scss

@@ -71,12 +71,23 @@
   &-section {
 
     @extend %site-width-container;
+    margin-bottom: $gutter-half;
 
     h2 {
       @include bold-24;
       margin: $gutter-half 0 10px 0;
     }
 
+    h2 {
+      @include bold-36;
+      margin: 0 0 $gutter;
+    }
+
+    .with-keyline {
+      border-top: 1px solid $border-colour;
+      padding: $gutter * 1.5 0 0 0;
+    }
+
     img {
       max-width: 100%;
       margin: 0 0 $gutter * 1.5 0;