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/templates/components/email-message.html

@@ -31,8 +31,10 @@
         </tbody>
       </table>
     {% endif %}
-    <div class="email-message-body">
+    <div class="email-message-body" data-module="expand-collapse">
+      <div class="email-message-body-wrapper collapsed">
       {{ body|nl2br }}
+      </div>
     </div>
   </div>
 {% endmacro %}