This branch of changes will also include the slash commands support, and probably some other updates for newer Discord features.

Refactor to pass linters

The bot is now using slash commands, with the implicit "/" command
prefix, so we don't need a config option for it anymore.

Pycord 2.0 understands naive timestamps as local time, and can handle UTC aware timestamps, so we can simplify the code paths.

Change type from int to float, so that values <1 hour can be used.

Lower the value to 12 hours, since 16 seemed too long in practice, with players queueing in the evening, and still being queued during the following day's activity.

Add database support for PostgreSQL and Sqlite3, refactor stuff to modules. Wip.