Join/Leave Message, Remove Modmail API

Changed

All support for Modmail API (api.modmail.tk) has terminated.
If you're still using api.modmail.tk, you will need to migrate to the self-hosted database
option ASAP. Your bot will not work unless you switch to the self-hosted option. Refer to the installation tutorial for information regarding self-hosted Modmail.

If a member leaves/joins (again) while they are a recipient of a thread, a message will be sent to notify you that this has occurred.