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 branch 'master' of github.com:alphagov/smart-answers
Burundi
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.
Format dates and salaries in previous answers
Temporary fix for upcoming user research until smartdown PreviousQuestion
returns an Answer object on which we call the humanize methods
Last SPL snippet changes before user research
Last SPL question changes before user research
Disable scroll to previous answer for SPL case
Temporary fix for user research workshop
Merge branch 'master' of https://github.com/alphagov/smart-answers