fix(image-404-handling): handle 404 from other sites

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Fixes an issue with #3811 – it's written to only handle images hosted by the site. This PR fixes it, so any not-found image will be replaced by the fallback (default) image.

This is an alpha-hotfix, since the changes are already on alpha branch.

How to test the changes in this Pull Request:

1. Repeat testing steps from feat(media): enable setting a default image for 404 images #3811, but also use a broken image link to a different site. https://nothing.here/image would suffice.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[adekbadek]

PHP tests are failing today for code added in #3484. Something weird about running this code on this particular date. Not related to changes in this PR, these tests also fail on another PR submitted today (#3871).

[naxoc]

Tested this and it works with images from different domains. 👍

[matticbot]

🎉 This PR is included in version 6.2.0-alpha.6 🎉

The release is available on GitHub release

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