Refunds & payments administration overhaul

[nbusseneau]

As a user
I want to be able to ask for a refund
So that I don't have to find a contact address, send a mail, and pray for a quick answer (or send a Facebook message, or smoke signals or something).

As a treasurer
I want to be able to see the list of user-asked refunds and confirm them
So that I don't have to do it manually through mails and the database interface.

As a treasurer
I want to be able to manually register a payment
So that I don't have to do it manually through the database interface (especially useful for "Amicaliste" fares).

[nbusseneau]

See http://old.insalan.fr/admin/paiement/ and http://old.insalan.fr/admin/remboursement/ for basic examples.