feat: add bot commands for tournament management

[Fruktus]

Provides very basic tournament management plane.

It includes:

• Creating
• Starting
• Joining
• Leaving
  a given tournament

It does not include:

• Recognizing banned users and preventing them from joining tournaments
• Cancelling (or otherwise deleting) tournaments
• Reaction-based registration
• Match tallying
• Pre-signed matches

[Fruktus]

TODO:

• Tests
• Only one tournament will be active at a time - clean up the commands accordingly

Notes:

• We want to generate full tree when tournament is created (hence active_until can be None)