Refactor Gallery as nested image blocks

[antonis]

gutenberg: WordPress/gutenberg#32382
gutenberg-mobile: wordpress-mobile/gutenberg-mobile#3579
WordPress-Android: wordpress-mobile/WordPress-Android#14769

Sub PRs:

• [Gutenberg] Gallery refactor - Update upload completion processor: [Gutenberg] Gallery refactor - Test upload completion processor #16721

Description

This PR brings in the refactored Gallery block with inner image blocks. No actual functional code was needed for this. Only tests were added to validate that the current upload completion processor can handle the new refactored gallery.

To test:
Check #16721

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   Check [Gutenberg] Gallery refactor - Test upload completion processor #16721

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading it from AppCenter here with build number: 52368. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

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[mkevins]

Approving as already reviewed and tested via the only "sub PR". Thanks Antonis!