Update colours

* Remove legacy `tone-` palette values (update to latest pasteup-palette)
* Update yellow/orange in comment-related SVGs to match latest palette value
* Remove `colour()` mixin (stop abstracting the abstractions!)
* Saner colour variables (reduce amount to those actually used/needed)
this was intended to be the replacement for our current colour system. it's much more succinct than the guss-colours mixin, removes the extra layer of creating variables and creates a single way for us to use use colours. 

I just haven't had a chance to speak to everyone about it or do a refactor