Add 'monitors' to users to track certain events

[sciwhiz12]

Occasionally, it is useful to be alerted whenever a certain user does some action, such as joining the server, leaving the server, writing a message, or so on. One concrete example would be to alert the moderation team when a user rejoined the server after a successful appeal which was conditioned on some action being done on their return.

This would be a useful feature for Camelot to implement, to avoid having to manually track users and their actions (which would occupy a lot of time and energy for any moderator to do).

This could be considered as an 'extension' of Discord AutoMod's ability to detect specific words, phrases, or match custom regexes, and send an alert to a configured channel, but working for other events like user join, user leave, and so on.