jetpack-mu-wpcom: Enable Classic block inserter support via sticker

[yuliyan]

Proposed changes

• Allow sites with the enable-classic-block-inserter-support sticker to opt in to Classic block inserter support.
• When the sticker is present, add the wp_classic_block_supports_inserter filter with __return_true.

Related:

• Disable TinyMCE: Repurpose experiment as Classic block removal WordPress/gutenberg#77838
• Disable Classic block: Control inserter support via filter WordPress/gutenberg#77845

Previous attempt to allow enabling Classic block support:

• jetpack-mu-wpcom: enable gutenberg-no-tinymce experiment for all sites #48317

Related product discussion/links

phcsdm-1cI-p2#comment-1711

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

No

Testing instructions

• Use a site running a Gutenberg build that includes the wp_classic_block_supports_inserter filter. If the filter is wrapped in an experiment, make sure the experiment is enabled.
• Confirm the site does not have the enable-classic-block-inserter-support sticker.
• Open the block editor for a post or page.
• Open the block inserter and search for "Classic".
• Confirm the Classic block is not available for manual insertion.
• Add the enable-classic-block-inserter-support sticker to the site.
• Reload the editor.
• Open the block inserter and search for "Classic".
• Confirm the Classic block is available and can be inserted.

[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 WoA dev 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 (WordPress.com Site Helper), and enable the enable/classic-block-inserter-support branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin enable/classic-block-inserter-support

Interested in more tips and information?

• 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]

Thank you for your PR!

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/jetpack-mu-wpcom/src/class-jetpack-mu-wpcom.php	2/354 (0.56%)	-0.00%	2 ❤️‍🩹

Full summary · PHP report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}

[tyxla]

LGTM, with a minor fix

Let's merge after GB 23.1 has been released.