request: automatic username confusable cleanup

[kyranet]

Is your feature request related to a problem? Please describe.

Sometimes, users join a server with confusable usernames which are both hard to mention and annoying.

Describe the solution you'd like

A system that uses confusables to clean up usernames. This might need to take #2199 into account to not run into nickname conflicts.

Describe alternatives you've considered

N/a.

Additional context

#222 in #suggestions