[Email]: Transactional HTML email: failed one-time payments

[ssandino]

Is your feature request related to a problem?
Currently, if a payment fails we are not communicating with the potential contributor and helping him to solve the issue (customer retention strategy).

Scope
The scope of this issue is only the failed one-time payment of a new user (not failed payments of existing contributors). The failed one-time payment of a new contributor is the simplest case and can serve as exemplary case (being also the first transactional email on the public repo).

Describe the solution/feature
Every new contributor who made a payment that didn't get through, should receive a reminder email a day later (24h after payment failed) to fix the problem. The email should be personalised and offer additional options (bank wire, twint link) for the payment.

Technical p
The Stripe API should provide us following data for failed one-time payments:

• status = failed
• amount
• currency
• email or receipt_email
• name (here we have an issue, as Stripe provides fullname not firstname, which we use in email salutation, therefore we might have to delete the salutation in the email. Gender is also missing, which is important for salutation in some languages. However name is still important and we should use it in preview text or in relation with email for not being marked as spam)
• failure_code
• failure_message (e.g. Your card has insufficient funds.) Even better would be the official description of the decline code, which can be found here. I didn't find them in the API, but the description could be matched with the decline code which is provided by the API.

Design proposition

See PR: #157

Note that this email also features our new font Unica77:

[ssandino]

Adding the payment link from Stripe would be nice