I'm realizing the trials require some additional analysis to be made, e.g. what happens to the first recurring payment when the trial has not been paid? Use case:

• Buy a course with a trial of 1 year at 1$ and 10 months of recurring payments of 10$ each
• Select the manual payment as gateway and complete the order
• Never actually pay the fee
• After one year the trial ends, you never paid for it, the order is still in pending payment
  You can potentially still go to your account to switch the source, but what should you be pay now? Ideally both the trial and the first recurring payment? But no it's not how it works right now, only the first recurring payment fee will be displayed, and with this PR only the trial fee instead will be displayed.

When you switch the source, because of the issue #2305, all the gateways, as of now, will ask you to pay the first recurring payment, basically skipping the payment of the trial, and the next recurring payment will be calculated based on the recurring payment rule, so 1 month starting from:

• if the trial has not ended yet: from the trial end date .
• if the trial has ended: from the last successful transaction (the one you just paid) if the stored next payment date for the order is passed (as in the case, because the stored next payment date is the first recurring payment date which is the trial end date).

I'm thinking that when the trial ends if you haven't paid it no recurring payment should ever be requested and the order should go to "failed" status, or at least you shouldn't be able to switch the source at all. I think that we should prevent source switching for orders with unpaid trials when the trial ended, and we'll be fine.

I'm turning this PR to a draft while we think to a better solution.

I'm going extrapolate the performance improvements from this and turn that part into a new PR.

Added to Q4 ideas