Add discount on next billing cycle

[birkjernstrom]

Ability to add a discount to an upcoming billing cycle for a customer, e.g in case of issues or as a reward (feedback/bug reports). Requested by lindesvard on Discord.

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Polar Context: We're gradually building the whole billing engine ourselves with each feature we're developing these days, e.g usage based billing. Once we have migrated it will be a lot easier for us to build features like this vs. tie it to a specific underlying PSP, e.g Stripe. So in transparency, I don't think we'll get this feature done this year, but hopefully early next with our own billing engine this is easy to do.