Enable filtering by group facet via API without city

Hi, and thank you for maintaining the radar.

I noticed the results on the API for a search with the group facet don't match the intended group. Adding the city facet to the request gives the right results.

See for example here vs here.

Here's what tech@ replied via e-mail:

But the group facet isn't enabled unless there is a city selected so https://radar.squat.net/en/events/group/143808 I think there might be an issue to fix that somewhere

I feel like since the group ID is unique, it should be usable for the API to return events on its own without knowing the city associated.

What do you think?