add support for /host

accessing /host will return all the available hosts.  This also makes
the behavior of the "Hosts" navigation menu match that of the
"Playbooks" menu: it's limited to 10 items at most, with an "All
hosts..." link at the bottom.

These limits can be controlled by the `NAV_MENU_MAX_PLAYBOOKS` and
`NAV_MENU_MAX_HOSTS` configuration options in `app.config`.

ara/templates/navbar.html

@@ -17,6 +17,7 @@
             {% for host in hosts %}
             <li><a href="/host/{{ host.name }}">{{ host.name }}</a></li>
             {% endfor %}
+            <li><a href="/host">All hosts...</a></li>
           </ul>
         </li>