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)
I personally think `$c-news-button-text` or (whatever pattern we want to use) is more readable than `$c-neutral-9` or `colour(live-main-2);`