Optimise PNG images with ImageOptim

The savings are as follows:

| File                             | Saved           | Percentage |
| ------- | -------- | ------ |
| public/chosen-sprite.png         |   108/646 bytes |        16% |
| public/chosen-sprite@2x.png      |   134/872 bytes |        15% |
| public/docsupport/chosen.png     | 1958/3467 bytes |        56% |
| public/docsupport/oss-credit.png | 2309/6709 bytes |        34% |

(ImageOptim is available [here](http://imageoptim.com/))