Introduce concept of person 'label' to indicate adult/child categorisation

[kws]

The Figma designs call for an adult/child label on the search results page - as the rules around this aren't clear and may vary between authorities, introduce generic concept of label that can be tailored to custom use.

[kws]

@saraguerreirodesousa do you think a generic mechanism where the rules are implemented in the back-end is sufficient to ensure good usability without causing all of the data integrity issues we discussed?