Pemula add subnav

 Overview Template with Sub Nav

1. add sub nav at header. it will be similar to blog template. should
support mobile view. this sub nav will show its child pages
2. add external links at sidebar. if no external link configured for
that page, then hide the widget

How To Use
1. Create a new page using "Overview with subnav template" template.
2. Add the external links as custom fields. The custom field name should
be "External Link" and you can add multiple fields with this name.
3. Nav should read child automatically
4. We can check the behavior of the sidebar if no
"External Link" are added.