REF Refactor to reduce duplication on payment forms

[mattwire]

Overview

Share a little bit more code, reduce duplication

Before

Duplicated code for frontend/backend forms. Some called twice.

After

Shared code for frontend/backend forms. Less called twice.

Technical Details

Comments

[civibot]

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[eileenmcnaughton]

@mattwire this makes sense to me as the code is centralised more.

Bigger picture what should we do? I feel like maybe the payment form functions should be in a trait?