Separate moderator logs from event logs

[MatthewL246]

Resolves #34

Changes:

This creates a setting for a mod-log channel, which will log moderator events like punishments and slow-mode configuration. The event-log channel will continue to log user events like message updates and deletions.

Message deletions?

I originally wanted to move moderator-deleted messages into the mod-logs channel, but there are a few issues with this.

• There is not an easy, reliable way to tell if a message deletion event was done by a moderator or a user. The standard way to check this is by fetching the most recent message deletion audit log event, but this is not fully reliable because the event does not contain the message ID. It only contains the message author and channel where the message was sent, so the only way is to make a guess if the most recent event's author and channel ID match.
• This might make mod-logs too noisy to be useful, so I don't know if it is worth adding this.

This PR is a draft because a decision on how to handle message deletions has not been made yet. Moderator message deletions have been implemented, with certain pitfalls as noted in a comment.

• I have read and agreed to the Code of Conduct.
• I have read and complied with the contributing guidelines.
• What I'm implementing was an approved issue.
• I have tested all of my changes.

[wolfendale]

LGTM

[jonbarrow]

I do agree with your original message about this not being super great. This looks like it would be very error prone in large servers like ours where tons of messages are deleted all the time. I'm not sure this is worth keeping

[MatthewL246]

I've gone back and forth on this. I think that it is worth trying out, but if the edge case is hit often enough that it causes a significant amount of confusion, it should be removed. I don't know how common it is for users to delete their own messages within 5 minutes of one of theirs getting deleted by a mod.

[CLAassistant]

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