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)
Agreed, `colour()` is better than the variables (and certainly clearer and more direct than the `$c-xxx` variables). Will add back in… it’s just a pain (as always) having two schemes running concurrently.