Add Repayment Frequency for Legacy Loans

Some Legacy Loans don't have a repayment frequency. We're to treat them as monthly repayments, instead of doing a migration this commit just adds another RepaymentFrequency. This way we don't loose any data if we want to do something different in the future.

Fixes "undefined method months_per_repayment_period for nil" error caused
by old loans having a repayment frequency id of 0