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)
yeah @sammorrisdesign and I have talked about `#ffffff` a lot, wondering whether to give it a name. only value i can see in it is we ever wanted to change ‘white’ to slightly off white or something, and do that across the site. Not sure if that would ever happen though. And also, maybe that would be the time to do it?

it’s a weird one though