Rework NAT settings

Support static NAT for L2 server networks. Also some other minor
tweaks.

web/templates/index.html

@@ -4,6 +4,7 @@
 <ul>
 {% if current_user.is_admin %}
 <li><a href="{{ url_for('config.index') }}">Nastavitve
+<li><a href="{{ url_for('config.edit', name='networks') }}">Omrežja
 <li><a href="{{ url_for('config.edit', name='groups') }}">Skupine
 <li><a href="{{ url_for('config.edit', name='forwards') }}">Luknje
 {% endif %}