malcontent-control: Add link to guidance on how to set parental controls

Try and nudge parents/carers towards the best practice for how to set parental controls on a user, by linking them to appropriate external content from people who know what they’re talking about.

This external content can vary in the translations so that parents are pointed to appropriate localised guidelines. In the UK, for example, this may be https://www.nspcc.org.uk/keeping-children-safe/online-safety/.

Signed-off-by: Philip Withnall pwithnall@endlessos.org