Add snake game

[haykam821]

This pull request adds a backend for a simple snake game. Closes #1.

Some considerations:

• Resolve segment collisions in a way that doesn't involve join priority
• Limit one snake per IP (or similar)
• Require authentication to join, and lock usernames
• Allow hosting multiple rooms
• Allow querying room information without joining the game
• Allow spectating a room
• Improve spawn positions (no spawning on edge)
• Prevent new snakes from killing snakes via their spawn position
• Remove snakes immediately when they disconnect

A client for this snake game backend was implemented in Taipan.

[Megadynamite]

This PR has been open and unmodified for three years, feel free to reopen when it is in active development again.