Consolidates all various usages of DOMContentLoaded event to external @wordpress/dom-ready package.

I checked that it's shipped in WordPress 5.1.1 at least, maybe even earlier.

domReady is super simple but it does a couple important checks in case the code runs after DOMContentLoaded event has already fired in case the script loads late. There are various ways to do this by checking readyState (or forgetting to check at all 😉) but like this we can ensure the same pattern everywhere.

This also saves a bit of code by sharing it from a package but that's not very significant. ;-) On the cons side it adds one network request but it'll get concatenated by caching plugins anyway.

Changes proposed in this Pull Request:

• Change Tiled gallery, Slideshow, Mailchimp and Recurring payments blocks to use wp.domReady. Enqueued dependencies for blocks are handled automatically so this will get loaded without manually adding it to anywhere.
• Change Photon.js to use wp.domReady and add package to dependencies.
• Fix previously unguarded use of document.body.addEventListener (document is in fact window.document)

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• Refactors existing features

Testing instructions:

Blocks

• Remember to build before testing (yarn build)
• Create a post with Tiled gallery, Slideshow, Mailchimp and Recurring payments blocks. For recurring payments block you'll need a site with paid plan.
• Look at the post from the frontend of the site — do blocks work as expected, are they interactive?
• Do pages with those blocks enqueue wp-dom-ready script? You can also confirm that _inc/blocks/BLOCK_NAME/view.deps.json file has wp-dom-ready

Photon

• Enable image CDN/photon
• Add an image to a post
• When modules/photon/photon(.min).js is loaded, does restore_dims function run? (maybe add console log to it to confirm?).

Proposed changelog entry for your changes:

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