Improve global modmail

[srnyx]

• [x] Webhook usage (user's name + avatar) (not doing anymore because webhooks can't reply to messages)
• [x] Message modification detection
  • [x] Edit
  • [x] Delete
  • [x] Reply
• [x] Use embeds
• [x] Close button (archives post) & Delete button (deletes post)
• [x] Expire
  • [x] 24 hours after inactivity (the last message sent must be from the user) send a message that pings the user to reply within 24 hours
  • [x] If they don't reply within 24 hours, the post will be archived