[FEAT] Warnings that require acknowledgement

[alyxw]

Overview

The bot should be able to send a warning and lock the user out of main channels and require acknowledgement of the warning, either by a react or typing "I acknowledge" before the user may continue participation in the server.
Their acknowledgement of the warning should be logged in their record. If the user leaves the server with a pending, unacknowledged warning they should be blacklisted from verification and it should be logged in their record.

The goal of this is to prevent users from later claiming that they weren't warned or making other excuses by requiring an acknowledgement.

Commands

[p]warn <userid> [warning]

DoD

• Add a warn command, which should lock users out of participating by removing channel read permissions. The bot should then send them a direct message informing them of their warning and requiring an acknowledgement, either by a react or by directly stating "I acknowledge". Their acknowledgement of the message should be logged.

• Add a listener to catch if a user with a pending warning leaves the server, and then blacklist them from verification and log it to their record.

Stretch

• Proposed ideas that aren't required but could be added with more discussion.
  n/a