Rewrite the supporting pages mixin.
Rewrite the supporting pages mixin.

Currently used only by Policy.

askl56/whitehall,askl56/whitehall,alphagov/whitehall,hotvulcan/whitehall,alphagov/whitehall,hotvulcan/whitehall,ggoral/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,ggoral/whitehall,ggoral/whitehall,alphagov/whitehall,robinwhittleton/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,robinwhittleton/whitehall