mod_register - registration_watcher - Support for deleted users

[ghost]

Hi!

In mod_register you can set registration_watchers to get notified when a new user registers.
Sometimes it would make sense to also get notified when a user deletes the account. Maybe there are even more account related events that would make sense to notify the registration_watchers.

This could be done via additional config option, e.g. notify_event = "register, deletion" or:

notify_event:
  - register
  - deletion
  - muc_creation
  - muc_deletion

The default would point to the current behaviour of just notify the registration watches of newly registered accounts.