Avoid resending Processing Order customer email when merchant wins dispute

[annemirasol]

Fixes #3603

Changes proposed in this Pull Request:

When the merchant wins a dispute, the order gets its pre-dispute status back. In some cases, the order goes back to processing, which causes the "Your order has been received" email to be sent to the customer again. We've received feedback that this causes confusion to the customer, who just lost a dispute and may not be very happy to begin with.

This PR adds logic to avoid resending the "Processing Order" email for the above-described scenario.

Testing instructions

1. Make sure your webhooks are enabled and working.
2. Install the email logging plugin of your choice, e.g. Post SMTP Email Log
3. As a shopper, add a physical product to cart and go to checkout.
4. Use 4000000000000259 as the CC number, and complete the purchase.
5. Wait a couple of minutes, and verify in wp-admin Orders that the order dispute has kicked in, and the order status has been set to on-hold.
6. View your email logs. Notice a few emails have been sent, including one Your <Store Name> order has been received customer email.
7. In the Stripe dashboard, simulate winning a dispute:
   • Go to the Disputes dashboard (https://dashboard.stripe.com/test/disputes), and view the dispute.
   • Click "Counter dispute"
   • Under "Why should you win this dispute?", choose "The purchase was made by the rightful account owner"
   • Under "Product or service details", choose "Physical product"
   • Click "Next"
   • Under "Supporting evidence", expand "+ Additional information" and type winning_evidence in the textbox.
   • Check "I understand..." and click "Submit"

8. Wait a couple of minutes, and verify in wp-admin Orders that the order dispute has been closed in the merchant favor, and the order status has been set back to processing.
9. View your email logs.
10. In develop, the Your <Store Name> order has been received email has been resent.
11. In this branch, the email was not resent.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[wjrosa]

Nice fix, @annemirasol! The code is good, and it works as expected. I can confirm the email is sent only once 👍