mark nav item as active for sub-pages

When navigating to e.g. a single blog page, the nav item this page is
in is now marked as active so users know in which area of the site
they are navigating at the moment.

layouts/partials/header.html

@@ -108,7 +108,7 @@
             <li>
                 <a href="{{ .URL | absLangURL }}" title="{{ .Title | default .Name }} {{- cond $is_search (" (Alt + /)" | safeHTMLAttr) ("" | safeHTMLAttr ) }}"
                 {{- cond $is_search (" accesskey=/" | safeHTMLAttr) ("" | safeHTMLAttr ) }}>
-                    <span {{- if eq $menu_item_url $page_url }} class="active" {{- end }}>
+                    <span {{- if hasPrefix $page_url $menu_item_url }} class="active" {{- end }}>
                         {{- .Pre }}
                         {{- .Name -}}
                         {{ .Post -}}