Fix bug preventing redemption of gift subscriptions

[Moggach]

Description

This PR adds an additional attribute check for applies_to attribute on coupon object returned from Stripe before checking for which products (if any) the coupon applies to.
This prevents the app from crashing due to an attribute error

Motivation and Context

Addresses issue LBC-404

Motivation and Context

Download the branch and run locally
Create a gift subscription using Stripe test cards
Copy gift card code
Sign out and go to http://127.0.0.1:8000/redeem/
Enter the code, create another account and add shipping details
The gift card should be redeemed successfully

How Will This Be Deployed?

Normal deployment process

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: app/utils/mailchimp.py

Function	Unhandled Issue
mailchimp_contact_for_user	ApiClientError /checkout/{price_id}/{product_id}/ Event Count: 21

📄 File: app/utils/stripe.py (Click to Expand)

Function	Unhandled Issue
create_gift_recipient_subscription	AttributeError: applies_to /redeem/setup/ Event Count: 5

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[joaquimds]

looks good to me, although you could also add:

if (
  hasattr(promo_code.coupon, 'applies_to') and 
  hasattr(promo_code.coupon.applies_to, 'products') and 
  product_id in promo_code.coupon.applies_to.products
):