Open links to documentation in new tabs

Because users have difficulty getting back to the Notify admin
interface.

The `rel` attribute mitigates against [a nasty cross-domain
vulnerability](https://mathiasbynens.github.io/rel-noopener/).

app/templates/views/api/documentation.html

@@ -22,7 +22,7 @@
       ('Ruby client', 'https://github.com/alphagov/notifications-ruby-client')
     ] %}
     <li>
-      <a href="{{ url }}">{{ name }}</a>
+      <a href="{{ url }}" target="_blank" rel="noopener noreferrer">{{ name }}</a>
     </li>
     {% endfor %}
   </ul>