Added review queue page

Added a review queue page available at `/review`. It's pretty plain-looking right now, but it works. It looks like this:

![screenshot](https://f.cloud.github.com/assets/1644254/2291552/2b7033a6-a041-11e3-88dc-15c4a39600b2.png)

Implemented the logic for fetching the list of games pending review (sorted by wait time in the review queue) and for accepting / rejecting a game. Note that in order to access this page, you need reviewer or admin permissions.

Also added `lib/dates.js`, which contains methods for generating relative date strings used for the review queue wait time, and exposed a `relativeDate` nunjucks filter that uses this. It's not especially robust, but it works for the current use case. Included some unit tests and documentation too.