don't allow players to do /desolate_frontiers on top of each other

[choccccy]

@choccccy: potential high-cleverness solution to the issue where there aren't enough places to play DF, resulting in the main server's 3 gameplay channels sometimes all being occupied; if a DF menu is live (not timed out) in a channel, additional menus spawned in that channel should be ephemeral
just a hipfired, top of the head idea, but i think it might have merit

@udokao i've had a similar idea but instead of each new instance being ephemeral, we spawn an entirely new channel for the user which deletes itself on the DF menu's timeout

@choccccy: if we went this way, i'd throw that player in a (private?) thread instead, but yeah, also a viable solution