Add a billing email adress distrinct from the account email

That would be of a great help that invoices could be sent to an email dédicated to billing information so that our payement service could receive it directly.

Thanks @Equipe TDB! Would be great to add this as an option in the account settings in the future. For now, we can manually make this change for you if you contact us. See https://plausible.io/docs/download-invoices#changing-the-email-address-for-invoices