"Purge user messages" command

[vcarl]

We occasionally encounter situations where we need to remove a large number of messages from a disruptive member. It would help us out if we could use "User Commands" to add a "purge message" option in the context menu for a specific user.

• The command should be named "Purge recent messages".
• After invoking, it responds immediately with an ephemeral message asking how far back to purge. Can use same increments as the "delete messages on ban" UI from Discord. Default to 1 hour.
• After submitting duration, the bot should iterate through all channels and threads, scanning messages for ones from this user and deleting them.