Optimize images

This branch optimizes all images in Dashboard and Blockly by running [image_optim](https://github.com/toy/image_optim) from an EC2 Ubuntu instance using the following command from the root directory of the repo:
`image_optim --pngquant-quality 65-80 -r .`

image_optim performs lossless optimization on GIF, JPEG and SVG files using command-line utilities, and lossy quantization of PNGs using [pngquant](http://pngquant.org/).

As one example, blockly/static/skins/artist/avatar.png compressed from a 32-bit 477.86Kb PNG to an 8-bit 101.49Kb PNG with no perceivable quality loss.

This will drastically reduce the bandwidth footprint across all of Code Studio. However, we should manually look over the transformed images to make sure the optimized images load correctly in standard browsers. If there are any images in particular that need to remain untouched, we can revert them to their original version.