A/B Inline galleries

This A/B tests 50% of the users with the new style gallery trails, shown inline on a standard gallery page. It tests the engagement of users when presented with a little more of the gallery content before they engage with it.

There's also been some code refactoring to re-use the contact sheet outside of a regular trail.

## Current style
![inline-gallery-control](https://f.cloud.github.com/assets/518943/932507/7bbde7a4-0041-11e3-839f-edd4052b8d0e.png)

## New style
![inline-gallery](https://f.cloud.github.com/assets/518943/932498/4b7b0ab8-0041-11e3-9d3d-3ef19d2c2897.png)