Make carousel more robust against missing data

[sgomes]

The carousel currently fails if some crucial data properties are missing in the images it attempts to load. This will make it more robust, allowing it to fall back to other ones.

Note that the experience still won't be perfect, as the images will be opened with their gallery sizes (which are likely too small for the carousel), but unfortunately the carousel has no way of knowing the original, larger image size if the properties are missing.

This PR also restores the pointer cursor in some situations.

Fixes #11191.

Changes proposed in this Pull Request:

• Make Carousel more robust against missing data in images
• Make fade helpers more robust against missing elements
• Add pointer cursor to galleries that open the carousel

Jetpack product discussion

None. Related to a bug in this repo, however: #11191

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

No.

Testing instructions:

• Test with the sites in Carousel: error thrown in originalDimensions function #11191 (comment) and ensure the carousel works correctly. Note that these sites are on wpcom, so adjust your testing procedure accordingly.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello sgomes! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer and confirm D62990-code works as expected before merging this PR. Once this PR is merged, please commit the changes to WP.com. Thank you!
This revision will be updated with each commit to this PR

[github-actions]

Thank you for your PR!

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Jetpack plugin:

• Next scheduled release: July 6, 2021.
• Scheduled code freeze: June 28, 2021.

[jeherve]

This seems to test well for me. 🚢

[sgomes]

Thank you for the review, @jeherve! 👍

[github-actions]

Great news! One last step: head over to your WordPress.com diff, D62990-code, and commit it.
Once you've done so, come back to this PR and add a comment with your changeset ID.

Thank you!

[sgomes]

r227547-wpcom