(Phase 6) Validation for sum of debtor book coverage and debtor book topup

Ensures that the sum of the two fields do not exceed 100% (eg. 80% debtor book coverage and 30% debtor book topup).

Based off of the validations refactor.

[Diff with the actual changes](https://github.com/alphagov/EFG/compare/30d22edb88065f053c28f38cf77cb5fa6ca00592...12eb0b0de696e97cf32490115f876bf84d189186)