Refactor callable predicate to allow arity of 0

All callable predicates previously required an arity of 1, leading to things like `->(_)` dotted everywhere.
Changed some of the predicates in marriage-abroad-v2 flow to take advantage of this simpler syntax.
Corrected the notice of marriage period for marriages in the same country as residency.
Merge pull request #1188 from alphagov/marriage_abroad_residency_and_notice_period_changes

Marriage abroad residency and notice period changes