Move site navigation to a partial

It can now be shared between the index and site header templates.

layouts/index.html

@@ -5,14 +5,5 @@
     <div id="content">
       {{ .Content }}
     </div>
-    {{- with site.Menus.main -}}
-    {{- $url := $.RelPermalink -}}
-    <nav class="site bulleted">
-      {{- range . -}}
-      <li><a class="{{ if eq .URL $url }}active{{ end }}" href="{{ .URL }}"><span>{{ i18n .Identifier | default .Name }}</span></a></li>
-      {{- end -}}
-    </nav>
-    {{ end }}
-  </div>
-</main>
+    {{ partial "site_nav.html" . }}
 {{ end }}