Improved admin registration filtering

[matthijskooijman]

Currently, I'm missing two relevant features in the django admin for registrations:

• Filtering on multiple values (e.g. registrations for two events, or for multiple statuses). This is possible by changing the url (using foo__in=1,2), but not internactively.
• Filtering (and also displying) on event options. This is a bit tricky when registrations from multiple events are displayed, but once you select an event, this should be possible...

Maybe some of these help (from https://stackoverflow.com/a/69924979/740048):

• https://github.com/thomst/django-more-admin-filters
• https://github.com/modlinltd/django-advanced-filters
• https://github.com/demiroren-teknoloji/django-admin-autocomplete-list-filter

[matthijskooijman]

Filtering (and also displying) on event options. This is a bit tricky when registrations from multiple events are displayed, but once you select an event, this should be possible...

This was implemented in 7a2416b