Add pngquant process to the png-resizing service

This passes the output of our ImageMagik resize through to pngquant to produce lovely small PNG8 transparent images. This allows cutouts at 460px to be only 30kb in size which even our beloved @paperboyo is happy with.

As always i'm going to run this for a while in CODE to catch any issues before going to PROD.

/cc @robertberry @rich-nguyen 
As we discussed yesterday these are effectively blocking operations (or as I like to say "It's a `Future` not a `MiracleBlock`") so push up parallelism too (https://github.com/guardian/frontend/blob/png-resizing/png-resizer/conf/application.conf#L37-L38).

Or maybe just `import scala.concurrent.{Future => MiracleBlock}`