Added a function to age at date

[CloevVeller]

Added the function that checks the user's age at the time of the event in stead of at the time of enrollment

[SilasPeters]

Seems to work at first sight! However, our pipeline complains. You should see the logs of the pipeline. Perhaps some checks failed or your code does not match our style requirements.

[SilasPeters]

I would approve the changes, but I want to test this beforehand (dates and timezones might give unexpected results). I'm quite busy though, so don't expect me to have done this in a day or two. Perhaps someone else could adjust (or expand?) the test suite to test this edge case (and, especially, if no one else unjustly can sign up).