Ask # of years NI paid if DOB is within 4 months and 1 day

If the user's birthday is within 4 months and 1 day of the state
pension date we should be asking them to fill out more information.
The Ruby idiom is to have question marks for methods returning bools
Tell users they're too young if they're under 20 years of age
Further text amendments from Heinrich
Merge pull request #464 from alphagov/v2-calculate-state-pension-updates

V2 calculate state pension updates