Fees

[Changaco]

With Mangopay we computed the fees and added them on top of how much a donor wanted to put into their wallet, but with Stripe and PayPal we're unable to predict the fees because they're too complex, so when a donor sends 4 weeks worth of a donation the recipient actually gets 3.x weeks worth, and because of how the accounting currently works the donor is asked to send money again 3 weeks later instead of 4 weeks later. This is confusing and needs to be fixed.

• [ ] modify the code that tracks how much has been paid in advance to use the amount sent by the donor instead of the amount received by the donee
• [ ] document how fees work