Automatically generate SVG thumbnails for container layouts in Facia tool

Somebody who's slightly less ham-fisted than me when it comes to styling could probably make these look nicer, but it's cool that we can now automatically generate these, having moved so much display information into Scala.

![screen shot 2014-10-18 at 08 40 14](https://cloud.githubusercontent.com/assets/1001642/4688908/301408dc-569a-11e4-97bc-6b30e534a7cf.png)

Also we could think about improving the actual selector itself so that rather than exposing the IDs directly we have a UI component that an editor can select a thumbnail from. I'm too lazy to do that right now, though.