(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/loan-entry-validations-refactor...loan-entry-debtor-book-validation)

Depends on #220