Add clear refund policy to patron page

[dav1312]

Currently as far as I can see refunds are only mentioned vaguely in the terms of service.

we will generally give you a refund if you contact us within 14 days of the donation.

The refund policies are probably a very FAQ and should be in https://lichess.org/patron
Also, having clear deadlines like "no refunds after X days" instead of "generally [...] within 14 days" would be much better for users and the people deciding if a refund should be made.