Had to do a couple of changes to the database: add the currency_code to
the payment relation, to format the price according to the payment’s
currency instead of the company’s; and the reference SQL function, to
replace the equivalent golang function, so that i can use it to index
payments.
The rest is mostly the same as any other page, except that the
individual payment’s page is not a form, but a regular info dump.
I also moved the payment settings as a sub-route of payments, as i
believe this makes more sense than an additional user menu item.
