Implemented Map suggestions for Arena

I've found some maps which I would like to see on the server.

am_sewage: https://steamcommunity.com/sharedfiles/filedetails/?id=305324927
am_warehouse3: https://steamcommunity.com/sharedfiles/filedetails/?id=312854302
am_underground: https://steamcommunity.com/sharedfiles/filedetails/?id=324093714
am_xenon: https://steamcommunity.com/sharedfiles/filedetails/?id=644183353

Also I think some maps should be removed:

am_courtyard, the map is just way too small, it's not fun to play on this map.
am_minecraftbox, the map is really ugly in my opinion.