fixed images for sidebar-highlights

Previously, we were including a page excerpt but it wasn't being
rendered and so images contained in it weren't having their source
attribute expanded.

We now implement essentially a 'featured image' which is included by the
sidebar template. Cleaner for content editors and actually works.

To use, define "image" in the YAML frontmatter like so:

---
layout: post
title:  "Something of note"
date:   2013-07-15 19:01:00
categories: sidebar-highlight
image: /images/pic06c.jpg
---