Poster fixes

* Fixes #7261
* Fixes placed posters being visible on both sides of the wall.
* Fixes the poster_type var used to specify certain poster designs on the
map not working correctly, and makes the nuke op shuttle poster an
actual poster.
* Poster placement now belongs to the poster object instead of turf/simulated/wall
* Posters can now be placed on unsimulated and shuttle walls.