Add css/js polyfill for <details>

Use bool attribute open to change style when not supported. Listener to
change attribute and style to change visibility of children.
Import updated styles.
Add test for details support.
Fresh output
Update template to use <details> so it is collapsable.
New template for recent posts using <details>. Use in sidebar.
<br /> after each <details>.
Updated generated output
Surround each recent tag in a <div>