Add `/hasmap` chat command and support for extensions in `/vote map`

[GooberRF]

This PR:

• Adds the is_level_name_valid utility function, which takes a level filename as input (with or without extension), adds the .rfl extension if missing, and returns whether the server has that level installed.
• Adds the /hasmap server chat command (alias: /haslevel) - takes a level filename as input and outputs whether the map is installed on the server (and can therefore be voted for and played on).
• Implements the new utility function into the /vote map logic - adds support for specifying extensions when voting for maps. (previously, /vote map dm02.rfl wouldn't work, it will now)
• Adjusts the Vote passed message for change level votes to now print the level being switched to.

[is-this-c]

I do not really see why /vote map dm02.rfl should be supported. Who would use it? It is just simpler not to support file extensions imo.

Perhaps /has_map dm02 instead or /map_exists dm02 or /status map dm02. I like /check map dm02.

/status map dm02 or /check map dm02 would be aligned with /find map blunder if it were developed although I think it should be a console command.