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/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>