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 %}

app/templates/views/choose-template.html

@@ -42,7 +42,7 @@
         <div class="column-two-thirds">
           {% if 'email' == template_type %}
             {{ email_message(
-              template.subject,
+              None,
               template.formatted_as_markup,
               name=template.name,
             ) }}

app/templates/views/send.html

@@ -51,7 +51,7 @@
     {% endfor %}
   {% endcall %}
 
-  <p class="bottom-gutter">
+  <p class="table-show-more-link">
     <a href="{{ url_for('.get_example_csv', service_id=current_service.id, template_id=template.id) }}">Download this example</a>
   </p>