Add styling for thumbnail layout hints

Adds styling for the latest layout hint **thumbnail**, i.e. smaller than supporting and used a lot in reviews, travel and food sections.

- Thumbnail styling
- Remove background colour from ficaptions globally
- Make adjacent headings to thumbnails inline

### Screenshots

#### Mobile
![google chrome](https://cloud.githubusercontent.com/assets/80089/3736188/6da480ae-172b-11e4-9aa9-0c850f24425a.png)

#### Tablet
![google chrome](https://cloud.githubusercontent.com/assets/80089/3736189/6fb0daa0-172b-11e4-844a-374ce3f6d6c6.png)

#### Desktop
![google chrome](https://cloud.githubusercontent.com/assets/80089/3736190/7134fa00-172b-11e4-8c3a-4465179c6cd6.png)

/cc @mattosborn @rich-nguyen @theefer 

Just curious why you're targeting `figure` specifically?

(and I assume you add `.element` to override the specificity?)