refactor: Templatize donation amount form fields

The portion of the form with preset donation amounts, as well as the custom donation entry field, are used on both the Stripe/Paypal donation page as well as the BTCPay donation page. They use exactly the same markup and Javascript to render and operate, but until now, the actual markup used in each instance was a copy-and-pasted block of Jinja template code. This was cruft leftover from early development, and this markup is now contained within one external Jinja template (donation_amount_fields.html.jinja) which is included inline by both form pages.