We initially didn't do this for two reasons:
- Performance implications of calling the settings store for every event
that shows up in the calendar. As of d43e85da, we need that anyways.
- Performance implications of filtering in Python except SQL but... it
can't really be worse than not filtering at all.
- We don't easily know if it's valid for all events so we can't stop
rendering the unused calendar rows. That's an acceptable issue for
now, still better than nothing. We can always optimize later.
So we might as well implement it.
