feat: Send a message to an open ticket if a user leaves the server

[dlsnyder8]

Please describe the feature you'd like
Suggestions 516 & 510

• add on_member_leave catch
• check if member has open ticket
• Sends a message in the ticket saying the user has left the server
• Adds a button to message to quickly close ticket
• Invalidates button if user rejoins server before ticket closed

[chamburr]

This feature would mean that there might be messages from users that are not seen by staff...

[dlsnyder8]

I can add a toggle for it so that it's opt-in

[chamburr]

I mean, I don't like that there's a chance the intended recipient will never see the message. Users send messages through ModMail knowing that they will get to someone, and the role of ModMail is precisely to deliver the messages. While we don't care what staff do with the messages, some feature that makes those messages disappear is just unfriendly to the users. Ultimately, it's all about a balance between features for users and for staff, and for this one I'm leaning on the users' side.

[dlsnyder8]

If it was a premium feature, then the messages would not be lost since they would/could have advanced logging enabled. I agree with your point that lost messages aren't a good thing, but this only happens when the user who created the ticket leaves the server. It's a waste of staff time to write a response to a ticket (sometimes lengthy) only to be hit with a "Your message cannot be delivered."

The issue could also be modified to leave a message in the open ticket saying that the user has left the server and further messages can't be delivered. If that were the case I would also want to add an on_member_join trigger for if they decide to return so tickets aren't closed if the user joins back.

[chamburr]

Sending a message in the ticket is great. Could you update this issue?