Add a Blueprint for WP.org Live Preview

[brandonpayton]

Summary

This PR adds a Blueprint file so that WP Maintenance Mode can have a Live Preview button in the WP.org plugin repo. The Live Preview will load your plugin in the WordPress Playground web app using the provided Blueprint.

According to the WP.org docs:

There are two things required for a plugin preview button to appear to all users:

1. A valid blueprint.json file must be provided in a blueprints sub-directory of the plugin’s assets folder.
2. The plugin preview must be set to “public” from the plugin’s Advanced view by a committer.

Once those conditions are met, a Live Preview button should appear next to the Download button in the WordPress.org plugin repo, like:

Try this link for an example of the preview.

I work on the WordPress Playground project and have started creating some PRs to propose Live Preview for various plugins. If you have any feedback related to Playground or Live Previews, I would love to hear it.

Will affect visual aspect of the product

NO

Screenshots

A screenshot of the Live Preview blueprint running in WordPress Playground:

Test instructions

This is not a functional change, but you can try this link for an example of the preview.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[brandonpayton]

@HardeepAsrani, it looks like I was mistaken about where to place the blueprint.json file. I believe it belongs under the top-level assets directory for the plugin, under assets/blueprints and not under trunk/assets/blueprints.

Perhaps this should have been an issue with an example blueprint.json instead of a PR.