Consider date of birth at shift time, not at current time

[felixrindt]

Is your feature request related to a problem? Please describe.

When getting an indicator for minor participants in event detail, checking minimum age and in the youthwarden plugin, we should be sure to consider age at shift start time (and explain in a tooltip that that is what we do).

Describe the solution you'd like

The is_minor functions get a date kwarg for which to consider the age. callers use the date of shift start time in the tenants timezone to compute age.

Describe alternatives you've considered

We keep the current way of considering the current age. I think this is less what people want, esp. those waiting to turn 18. Any way, it should be consistent