Async and data integrity protections

[half-duplex]

• Write CSV using library
• Overwrite CSV with an atomic rename()
• Lock while checking/modifying/writing token list
• Command callback in a Task, since library doesn't seem to (??)
• Replace print()s with logging (and separate some verbose nio logging)
• Log of token reuse attempts and some other stuff
• User feedback for notice command
• Reinvite users sending their token again to the community, not just rooms
• Format using black, lint with flake8, check with github actions
• I've probably forgotten some stuff

[lovelaced]

Wow, looks great! Thank you!
We specifically do not want to re-invite users to a community because of a potential breakage in Matrix (ref matrix-org/synapse#2633 )