Update `MapVote` system to check `maps/` folder on server when adding a `Map` instance

[piqey]

Right now, we have them hardcoded in MapList. Sometimes, we decide we no longer want a map on the server, remove it from the workshop collection and forget about it (at least I do). Then, when I launch the server again and receive a map vote menu instance, I can receive removed/unmounted maps as choices in the vote menu.

I think I will:

• Change the constructor of the Map class to check the maps/ folder for the map name provided when SERVER is true
• Consequently, when SERVER is true, add a Map._mounted field and Map.IsMounted method, set to true if a matching map file is found
  • I'd be adding an additional check to the server-side MapVote logic to exclude maps imported from MapList that pass the Map.IsMounted test