[CPT][Reversal] Send an email when a charge is reversed, and include troubleshooting tips

> [!NOTE]
> This has been assigned to @kashsuks

A reversed card charge is when the merchant decides to not settle a charge. Here's what that means.
1. Cardholder makes a purchase
2. HCB approves
3. Merchant later reverses the charge

We often get questions about why a charge got reversed. These questions are ask when the reversal was not initated/requested by the cardholder.

When a card charge gets reversed, let's send an email to the cardholder. Also, include some troubleshooting tips.

The email should consider two paths:
- Reversal was requested by the cardholder. An example is the cardholder cancelled the order or requested a refund (before the charge settled).
- Cardholder is NOT expecting this reversal. The cardholder is currently confused, and we should provide troubleshooting tips.

In many cases, HCB is unable to detect a reason. A reversal state is solely determined by the merchant, and HCB has no say and no precise insight into why. We may have some hints, though: For example, if zip code didn't match, suggest reattempting with the correct zipcode. Sometimes, reversals happen because the Merchant's fraud detection determined that this charge is too risky/likely to be fraud.

In all the cases, HCB should recommend that the cardholder reach out to the merchant to resolve the issue.

I imagine the mailer will look something like this:

> Your charge of $100 on September 8th was reversed by MERCHANT NAME. Your balance has been updated to reflect this refund.
>
> **Wasn't expecting this reversal?**
> Merchants like MERCHANT NAME often reverse charges when an order is cancelled, a refund is requested, or for another reason at their sole discretion. To reattempt this charge, we recommend that you reach out to MERCHANT NAME to understand why they reversed this charge.
>
> **Troubleshooting tips**
> We noticed that the zip code you used on MERCHANT NAME does not match the zip code configured in your HCB billing address. Please reattempt with the address and zip code configured as your HCB billing address.

^ add links where helpful.