@dzucconi: Migrate old posts into new articles

This gets the migration script for old posts into Positron articles started. There's also some minor gardening things included in here.

Currently it's reading the post-related mongo collections from the Gravity database, mapping them into Positron articles, and bulk updating the Positron database. There's still some questions to be answered around this migration strategy, but this is just getting the ball rolling.

![image](https://cloud.githubusercontent.com/assets/555859/5074774/a49f2b2c-6e5b-11e4-97b3-a2c29e2d13fa.png)