Move site header and footer to the base template, and override the body block in index

layouts/index.html

@@ -1,16 +1,18 @@
-{{ define "main" }}
-<div class="platter grid">
-  <h1 class="site">{{ .Title }}</h1>
-  <div id="content">
-    {{ .Content }}
-  </div>
-  {{- with site.Menus.main -}}
+{{ define "body" }}
+<main>
+  <div class="platter grid">
+    <h1 class="site">{{ .Title }}</h1>
+    <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>
+      <li><a class="{{ if eq .URL $url }}active{{ end }}" href="{{ .URL }}"><span>{{ i18n .Identifier | default .Name }}</span></a></li>
       {{- end -}}
     </nav>
-  {{ end }}
-</div>
+    {{ end }}
+  </div>
+</main>
 {{ end }}