Add a prototype email template

If the templates page contains text messages and emails then there’s two ways it
could be structured:
- into two sections, all text messages first, then all emails
- emails and text messages interleaved, sorted by date

I think the second one is better. Imagine a situation where you mostly do emails
but have a few text messages. You’d have to scroll past the text messages to get
to your emails. Every time.

I reckon that the most commonly accessed templates will be the most recent ones.

app/templates/main_nav.html

@@ -6,7 +6,7 @@
     <li><a href="{{ url_for('.sendsms', service_id=123) }}">Send text messages</a></li>
     <li><a href="{{ url_for('.sendemail', service_id=123) }}">Send emails</a></li>
     <li><a href="{{ url_for('.showjobs', service_id=123) }}">Activity</a></li>
-    <li><a href="{{ url_for('.manage_templates', service_id=123) }}">Manage templates</a></li>
+    <li><a href="{{ url_for('.manage_templates', service_id=123) }}">Templates</a></li>
   </ul>
   <ul>
     <li><a href="{{ url_for('.apikeys', service_id=123) }}">API keys and documentation</a></li>