Slow mode slash cmd
Implement a moderation command that takes a duration, or some falsey value to set slow mode on the channel it's used in.
Require either the 'Manage channels' permission, or the staff role to use this command.
Log the action in the moderation channel, or system messages channel.
Next phase:
- Add a reason argument to the command
- Store event in DB, correlated with guild, channel, and moderator
- Add a command to retrieve all slow mode actions across channels, or for a specific one