Issue 2127: Most-shared/retweeted post

The first of the EOYs! The insight itself is pretty basic, but a couple things to note about the rest of this commit:

1. I created a new tpl (_posts_with_counts.tpl) for embedding posts w/counts (many of our EOY posts call for this). I didn't do anything w/CSS — it's just an H4 with a class of "count-header" for later styling. It requires that each post has a count property (like "20 retweets").

  ![_posts_with_counts.tpl](http://files.adampash.com/s/ThinkUp_1A02BE94.png)

2. I've added a conditional to insights.tpl for slugs matching "eoy"; on match, it adds two classes: "insight-eoy insight-wide". Figured insight-eoy can handle our eoy-specific styling.

I realize I took a leap with those things, and you may prefer another route — totally cool! Just thought I'd get the ball rolling.
I don't believe the crawler properly captures Facebook shares (it might not). If not, we'll limit this insight to Twitter.