Limit nickname changes

[CO2-code]

Description

Suggestion: Limit nickname changes to 2, similar to how most other games handle it.
Players get 2 free changes; after that, to unlock a third one the player should have a good reason or something...

Advantages:

1. Makes moderation easier.
2. Simpler than setting up nickname databases, bots, and other complex systems.
3. Eliminates the need to ban players who change their nickname more than 5 times within 24 hours.