Make PoliticalContentIdentifier a proper class
Make PoliticalContentIdentifier a proper class

Instead of passing around the edition instance amongst lots of class methods,
just make `PoliticalContentIdentifier an instantiatable class and make edition
an accessor.

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