This is to make sure that we don't leak keys in the argument and hence
in logs etc.

let more "fixed" parameters come first and then the dynamic ones so
that one can do currying with the immutable parameters applied.

Stripe developer dashboard provides a few keys and we specifically
need to use the secret key.

If for "some reason" Stripe servers returned a mismatched voucher
code, return http 500 status.

Mark the voucher as paid if the stripe charge API returns a Charge
object.

The skeleton is in place, a lot of actual work still needs to be done.

Also, explode the tuple that made its way into `redeemVoucherHelper`

`redeemVoucher' reads stuff from the db, makes decisions and then
writes to the db. However if there is another client doing a
simultaneous `redeemVoucher' for the same `voucher', then both of them
would redeem the voucher, which is incorrect. The operations should
have exclusive access to the db and this is achieved with
`withExclusiveTransaction`.

Vouchers table contain all paid vouchers, so to find if a given
voucher is unpaid, all we need to do is check membership in the
`vouchers' table.

Type class instances for `redeemVoucher` function has a lot of
code duplication. This change refactors them to reuse the main
logic of the code and abstract out parts that differ between
instances.