prevent roundstart arts on station z-level from being bombs

[aft2001]

I'd say make this change exclusive to the RP servers. On Main, rounds are often short enough to the point where artbombs are just another one of the many things that'll end a round quickly, except only if nothing is done about it, so it isn't really an issue there.

However, the RP servers' rounds last about two hours on average, and artbombs are a fantastic way to just ruin peoples' plans and the round's potential. Station-wide or even just department-destroying devastation just isn't fun in a gameplay setting where you're more oriented towards player interaction than game mechanics.