(Phase 6) Variable premium and guarantee rates

With the introduction of Phase 6, premium rate and guarantee rate can now change based on Loan Type. In previous phases premium rate was typically 2%, guarantee rate 75%.

Phase 6's premium rate for Type F is now 1.2%, other types are still 2%.

Guarantee rate remains at 75% for Phase 6 but the ability to easily change this for new Phases is still needed.

Existing loans with a `premium_rate` and `guarantee_rate` on their record still use that value.