Warn when deactivating plugin while a media kit page is active

[peterwilsoncc]

Is your enhancement related to a problem? Please describe.

Deactivating the plugin while a media kit page is active will result in a broken design of the media kit page.

When a media kit page is published a warning should be displayed when deactivating the plugin to ensure site owners are aware that the page will be broken.

For bonus points, the check will evaluate the blocks of the publish page and bypass the warning if blocks created by this plugin are no longer used.

Designs

Something similar to the modal used by Distributor 10up/distributor#540

Describe alternatives you've considered

N/A

Code of Conduct

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