Warn the organiser when the maximum number of participants of an event is changed

Ghost User
2020-01-29T18:43:19+01:00

https://gitlab.science.ru.nl/thalia/

### Current behaviour

People on the waiting list don't get any notification and thus cannot know that their status changed.

### Expected behaviour

The organiser should do a follow-up.

### Steps to reproduce

1. Create an event
2. Enable x registrations
3. Register at least x+1 people
4. Change the number of registrations to x+1
5. See that the person x+1 didn't get an email notification