Factory refurbished PlayMusicOnMapLoad

To fix https://github.com/OpenRA/OpenRA/issues/6071 and bring it in line with the legacy https://github.com/OpenRA/OpenRA/blob/bleed/mods/common/lua/media.lua#L11 which this aims to supersede.
I still don't like the name ;)

Also, wouldn't it be better to make this default to `true`? Or is it on purpose that songs should only play once in a game?