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.
For the EOY insights, let's add a reusable InsightPluginParent::shouldGenerateEOYInsight($day_of_december) function, which will support running the insight on staging any day if it doesn't exist, and won't regenerate the insight on every crawl on the day this goes live. (I'll take care of this and swap it in here.)