Move site navigation to a partial

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

layouts/partials/header.html

@@ -5,13 +5,7 @@
     <h1 class="site">
       <a class="site-name" href="{{ `` | absURL }}">{{ partial "site_name.html" site.Params.shortTitle }}</a>
     </h1>
-    <nav class="site bulleted">
-    {{ with site.Menus.main }}
-    {{- range . -}}
-    <li><a {{ if $currentPage.HasMenuCurrent "main" . }} class="active"{{ end }} href="{{ .URL }}"><span>{{ .Name }}</span></a></li>
-    {{- end -}}
-    {{ end }}
-    </nav>
+    {{ partial "site_nav.html" . }}
     <nav class="social">
       {{ with site.Menus.social }}
         {{- range . -}}

layouts/partials/site_nav.html

@@ -0,0 +1,9 @@
+{{- $currentPage := . -}}
+{{- $url := $.RelPermalink -}}
+<nav class="site bulleted">
+{{ with site.Menus.main }}
+    {{- range . -}}
+    <li><a {{ if $currentPage.HasMenuCurrent "main" . }} class="active"{{ end }} href="{{ .URL }}"><span>{{ .Name }}</span></a></li>
+    {{- end -}}
+{{ end }}
+</nav>