fix: Fix BTCPay form

When the regression on the main donate form was fixed, allowing for proper validation of the custom donation field and the usage of the custom donation field as canonical donation value, the poor BTCPay form, which nominally renders the same donation-entry HTML that the standard donation form does, broke.

This is because the change to the donation form which fixed the regression needed to be propogated to the cryptocurrency donation form as well. To wit, the crypto form needed to have custom_donation attached to the form object instead of just painted directly onto the page with custom HTML, so it could be rendered by Django.

This change has been committed, and the BTCPay form has been tested and confirmed to work.