People should be able to pay what they want whenever they want, and it should add to the extension of their Membership.
- Paying more: shift next_payment_date of their membership
- Order should be generated on the fly when performing the payment (rather than at subscription creation time).
- Minimum initial payment is one period.
Reminder X days/weeks before expiry (maybe based on the period last paid for, e.g. when you pay for a month you get a notification one or two weeks before it expires, but when you pay for a full year you get it a month or two before it expires).(not part of this task; see T57675)
- Should also be available for automatic payment, as it just extends when the next automatic payment will happen.
- Assumes the price of a Membership is a constant amount per day, regardless of the originally chosen plan variation (e.g. price per month = price per year / 12, price per day = price per year / 365¼)