Infinite Scroll: ensure missing assets are loaded

[monsieur-z]

Fixes #12453

Proposed changes:

Let's consider a page containing a list of posts with infinite scroll enabled. As a user scrolls the page, a post item that's loaded on scroll and that contains a block may have missing styles. That's for instance the case of the Tiled Gallery block, whose layout is not respected in a list of posts.

Before	After

Images are stacked instead of being displayed as a grid

This PR ensures that, for posts loaded on scroll, the required assets are loaded.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

n/a

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Spin up a Jetpack site with this branch
• Install and activate a theme that supports infinite scroll, such as Dara
• In Jetpack > Settings > Writing (or go to /wp-admin/admin.php?page=jetpack#/settings?term=infinite), ensure that infinite scroll is enabled
• You may need to delete widgets in footers (at /wp-admin/widgets.php) to make infinite scroll actually work
• Create the necessary posts:
  • The fastest way is probably to ask me to export posts from my own test site; otherwise:
  • Create enough posts to have at least 2 pages of posts
  • Create a post that contains a Tiled Gallery block, and update its date so that it's rendered in the second page of posts
• Visit a page of the site that contains a list of posts, such as the home page or a category page
• Scroll the page
• Ensure the post item containing the gallery is properly rendered
• Optionally, test the PR with a simple and atomic site

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix/infinite-scroll-missing-assets branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack fix/infinite-scroll-missing-assets
  

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• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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[chrisbliss18]

I was able to confirm the original issue on trunk with the images not tiling. After updating to this branch, the images tiled as expected.

The change looks good and works as intended on my test sites.