Improve consent for underage subjects

Consent for children (given by their parents) expires when they come off age. There is a grace period starting with 16 years, but once they are 18 the consent should be considered void.

So far we only implement the first part, and that implementation is incomplete. The reason is that it is very hard to compare the consent date (default database) with the date of birth (contact database). We could of course duplicate the date of birth in both databases, but that would defeat the whole purpose of database separation.

The trick I used was to store whether a consent was given for an underage subject. This way I only had to compare the two dates when creating a consent, not when doing a query. Still this is redundant information and I am not completely sure I got all the relevant places where a consent is written.

With this trick I was able to refactor the existing support and add the missing pieces.