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)
Maybe we should add white color to the palette too. Something like 'button-text-primary', 'text-secondary' or something like that? It is probably not going to change but...

Other than that :+1: