Telegram and Minecraft account binding

[vanutp]

Add a system to bind accounts using an external service (e.g. tgauth)

Features:

• [ ] Show user links in /list
• [ ] Add a command to get Minecraft username by Telegram account and vice versa
• [ ] Allow only users in the linked chat to join the server
• [ ] API
• [ ] Provide some kind of default implementation that will work out of the box
• [ ] Optionally show Minecraft usernames instead of Telegram names in the game chat + prefix/name decoration support