added apple pay domain association view

[danielfmiranda]

Description

Related PRs/issues https://github.com/MozillaFoundation/foundation.mozilla.org/issues/10262

This PR adds the following:

• A new environment variable APPLE_PAY_DOMAIN_ASSOCIATION_KEY
• A new view, apple_pay_domain_association_view which returns the contents of the env variable when the url /.well-known/apple-developer-merchantid-domain-association is visited
• Tests to make sure that the view returns the appropriate response with and without a key set

Checklist

Tests

• [x] Is the code I'm adding covered by tests?

Steps to test:

1. Checkout this branch and run docker-compose up
2. Visit http://localhost:8000/.well-known/apple-developer-merchantid-domain-association, the page should return the message Key not found. Please check environment variables.
3. Add a test string for the env variable APPLE_PAY_DOMAIN_ASSOCIATION_KEY
4. Restart the app
5. Visit http://localhost:8000/.well-known/apple-developer-merchantid-domain-association again, this time, the page should return whatever string you set for the env variable.
6. If everything is working as expected, testing is complete!

[danielfmiranda]

Hi @tbrlpld thank you for the great feedback on this PR!

I have updated the way we update the settings during the tests, and also updated the tests to use the assertContains method.