Truncate previews of email messages to ~3 lines

Emails can get very long.

When you’re trying to do other things on the page this results in a lot
of scrolling.

This commit truncates email messages to about 3 lines, and adds a JS
toggle which reveal the full contents of the email.

app/assets/stylesheets/components/table.scss

@@ -108,6 +108,7 @@
   @include core-16;
   color: $secondary-text-colour;
   margin-top: -20px;
+  margin-bottom: $gutter * 1.5;
   border-bottom: 1px solid $border-colour;
   padding-bottom: 10px;
   text-align: center;