Add an index page for the API integration

This commit adds a placeholder page which, for now, just has links to
the API keys page and links to the clients.

There’s more stuff to come on this page, but this commit just does the
reorganising so that it’s easier to review.

app/templates/main_nav.html

@@ -58,7 +58,7 @@
     <li><a href="{{ url_for('.manage_users', service_id=current_service.id) }}">Team members</a></li>
   {% endif %}
   {% if current_user.has_permissions(['manage_api_keys']) %}
-    <li><a href="{{ url_for('.api_keys', service_id=current_service.id) }}">API integration</a></li>
+    <li><a href="{{ url_for('.api_integration', service_id=current_service.id) }}">API integration</a></li>
   {% endif %}
   </ul>
 </nav>