Phase 6 variable premium and guarantee rates

**Pull request contains `phase-6` at the moment.**
You can diff just the [changes in this branch](https://github.com/alphagov/EFG/compare/phase-6...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.