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/
Posters can now be placed on unsimulated and shuttle walls.
Merge remote-tracking branch 'upstream/dev-freeze' into poster

Conflicts:
	code/game/objects/effects/decals/contraband.dm
Merge pull request #7276 from mwerezak/poster

Poster fixes