Notify registration managers when a participant withdraws their registration

[kewisch]

Is your feature request related to a problem? Please describe.
Registration managers, or those who are in the notify list for specific reg forms, get notified on various state changes. However, it appears there is no notification to managers when a registration is withdrawn by a participant.

Describe the solution you'd like
An email is sent to registration managers, and registration form notify emails, when a user has withdrawn their registration.

Describe alternatives you've considered
Looking at the list once a day to see who withdrew? ;-)

Additional context
It looks like notify_registration_state_update is not called in RHRegistrationWithdraw, while it is in RHRegistrationManageWithdraw. Would it be as simple as adding the call here?

[ThiefMaster]

I haven't tested it, but I think this is exactly what's missing there and would be exactly what I'd do to fix it (in fact I looked at the code and only then saw your last part with the proposed solution ;)).

A PR would certainly be appreciated if you'd like to! :)