Placing a poster keeps the serial on the roll. Fixes #7598

This changes `/turf/simulated/wall/proc/place_poster` to be more like `/obj/item/weapon/contraband/poster/afterattack`, which fixes the poster's serial being changed while being placed. I'm unsure why there are two procs, so I'm leaving that as-is for now. All I know is that this fixes #7598.