`assets/assets.map` and `assets/head.min.css` are the last remaining coupling between the Grunt and SBT builds. Grunt compiles them into the Play common conf directory for us to load at runtime to know where assets are. I don't fancy loading these from S3 and explaining why I did that after a failure.

The reason `assetMap` is passed in is because it is tested. I don't think I want the template to be telling `Assets` where Grunt compiled the minified head stuff.