Add premium_calculation_strategy to PremiumSchedule

One time data migration sets premium_calculation_strategy to
'legacy_quarterly' for all existing premium schedules.

New premium_schedules derive the calculation strategy from the repayment
frequency of the loan.
Fix typo in request spec
Implement correct quarterly loan repayment premium payment schedules

Identical to the legacy logic except for the additional final payment
on non-exact quarter number repayment durations.