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.
Had to change setup_redsys because admins can not read the current
encrypt key, thus it is not possible to `set encrypt_key =
coalesce(…, encrypt_key)`.
Not that it did much sense, anyway, as i was already inside the branch
of the if when encrpty_key is null.
However, it seems that this also affects in the `on conflict` update. I
assume this is because `excluded` is some kind of row of the relation
and has the same restrictions.