Carousel: Only query posts with thumbnails

[obenland]

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:

Not sure how smart it is to unilaterally open the posts endpoint up to meta queries. I suppose we could pass a custom parameter and "hard-code" the post thumbnail meta query based on that?

Regardless, with this change the carousel in the Editor will now contain the same posts as it does in the front-end. Currently it just displays X number of most recent posts and hopes they have featured images assigned to them.

How to test the changes in this Pull Request:

1. Add the Carousel block in the editor and play with the post count attribute.
2. Make sure the carousel contains as many posts with thumbnails as you selected in the attribute
3. Double-check that posts on the Editor side are the same as in the front-end

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?

[jeffersonrabb]

In my testing I'm getting identical results on this branch and master. Am I testing correctly?

• Added a console log to view latestPosts to verify what's coming back, after https://github.com/Automattic/newspack-blocks/blob/master/src/blocks/carousel/edit.js#L46
• Removed the Featured Image from top post.
• Added and published a new post with no Featured Image.
• Set Number of Items to 3
• On master and update/thumbnail-requests I'm getting 3 posts back, including the two featured image-less ones.

[obenland]

@jeffersonrabb I updated the callback to be more specific about the parameters it expects and acts on. It should now only return posts with associated featured images.

[frontdevde]

I did a functional review and can confirm that there's now the expected difference in behavior between master and this branch:

master

Editor	Frontend

this branch

Editor	Frontend

Steps I took to reproduce the issue on master:

• created four posts with featured images
• removed the featured image on the fourth post
• created a fifth post without a featured image
• test with number of items set to 2 and 3

In the above example, there are three posts that could be shown. On master, in the editor they don't. On this branch, they do.

[jeffersonrabb]

🚢

[matticbot]

🎉 This PR is included in version 1.3.1 🎉

The release is available on GitHub release

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