Obtaining a receipt from a donee for a tax deduction

[Changaco]

This issue is both an alternative to and a prerequisite for #383. Instead of going all the way to the automatic generation and storage of the tax compliant receipts by Liberapay, it stops at enabling a donee to generate the receipt and send it to the donor themself.

• modify the www/%username/edit/countries.spt file to enable an organization to specify which countries they can provide tax-compliant receipts for
  • save the information in a new recipient_settings.tax_deductions column
• when a payment has been received, ask the donor whether they want to request a tax-compliant receipt for one of the supported countries
• if they do, present them with a form containing the information that will be sent to the donee: email address, full name, postal address, tax/registry number if the donor is also an organization, and a final text field for any other information or message to be sent to the donee
• send an email to the donee when a donor submits a request