migrate css to scss and use hugo pipe to process

[DaveParr]

This is a prototype to use scss pipes in hugo to supply styling based on https://gohugo.io/hugo-pipes/introduction/#go-pipes

This works because

• css can be scss just by renaming the file
• hugo has inbuilt ability to process scss out of the box https://gohugo.io/hugo-pipes/scss-sass/
• hugo expects to find the scss in the assets folder https://gohugo.io/hugo-pipes/introduction/#asset-directory

This change should be matched by changing the config.toml so that where it currently sources a css file as a 'custom css' which is passed into the head.html it should instead do nothing, but leave that open to the user if they would like to use it instead.

This foundation should allow some very aggressive refactoring of the css mess #42