Add responsive image component

[gmaclennan]

Displaying images could be more efficient. We use our own resizing service which is running on http://resizer.digital-democracy.org/ to get thumbnails and previews. We need an <Image> component that uses [react-dimensions](https://github.com/digidem/react-dimensions] to choose the best size to request. Our resizer currently whitelists certain domains - we should detect 403 errors and just load the original full-size image in that case.

We could speed things up more by keeping track of sizes of image that have been loaded and when requesting a larger image show a smaller one that will be in the browser cache first whilst the larger version loads.