Conditionally add extra monthly pay date

Where the pay date in the month is after the pay end date (39 weeks from leave start)
an extra pay date is required to pay the remainder.
Conditionally add paydate for a certain day each month pay pattern

If the last SMP paydate falls before the SMP end date the remainder
should be paid the following month in line with the pay pattern.
Remove redundant comparison
Merge pull request #443 from alphagov/maternity_monthly_pay_updates

Maternity calculator monthly pay updates