Create plugin support for CDN'ing of static assets.

[georgestephanis]

Add in CDN hosted versions of Core / Jetpack scripts and styles to speed up sites and better cross-site cacheability.

To do:

• Add caching to external http calls so they're not requested on each load.
• Abstract out plugin script/style swapping to enable for other plugins (like WooCommerce) as well.
• Confirm RTL styles are handled correctly.

Testing instructions:

• Activate Photon CDN through the old module settings. Confirm that scripts / styles are now pointing to c0.wp.com resources in the page source. If testing using a development version, you can use a filter to override versions to production versions:

add_filter( 'jetpack_cdn_plugin_slug_and_version', function( $values ) {
	return array( 'jetpack', '6.5' );
} );
add_filter( 'jetpack_cdn_core_version_and_locale', function( $values ) {
	return array( '4.9.7', 'en_US' );
} );

• Make sure everything loads in a couple browsers.
• Test in multisite
• Test in subfolder installs.
• Test in RTL

Proposed changelog entry for your changes:

• Speed up sites and increase max concurrent connections through Photon by cloud-hosting Jetpack and WordPress Core scripts, styles, and assets in addition to content images.

[jetpackbot]

That's a great PR description, thank you so much for your effort!

Generated by 🚫 dangerJS

[georgestephanis]

I'm unsure if it may be wiser to change how we swap the urls and such to a just in time based off a later filter or action.

[georgestephanis]

I'm probably okay skipping exceptions for the emoji ones for the moment -- I don't want the initial PR to be too epically complex.

Good catch on Akismet -- I think that's because it's coming back with the core hashes, as it's bundled in the zip -- but not getting pulled over by @bazza's cdn. Not a big deal, I'll just prune them from what gets rewritten.

Talking to @claudiulodro about WooCommerce in Slack currently, seeing when we can get WC core onboard. They're currently past feature freeze, so it'll need to probably be for 3.6

[zinigor]

@georgestephanis sorry, there seems to be a problem with your latest change. It looks like Jetpack, and maybe some other plugins as well, enqueue assets with URLs like this:

We're going to be breaking these for all plugins that rely on that mechanism. We could probably blacklist URLs that have admin-ajax in them, but that sounds risky because other plugins might do something similar with a different URL. What's your opinion on this?

[georgestephanis]

Ahhh shucks. Okay. Good find, I'll add a check.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Please review this diff before merging: D18161-code. (updated diff)

[georgestephanis]

That should have it fixed, @zinigor

[zinigor]

Tested, fixes the problem, thank you! And huge props for @dereksmart for finding the problem!

[jeherve]

I tested the PR one more time, and did not run into any major issues. It works well.

One quick note is that you can't really test this when using the Lazy Images feature; since we've modified Lazy Images in the past few weeks, the .js file served from the CDN is not compatible with the current development version of Jetpack.

[georgestephanis]

Once it's launchable, I'd like to switch it to only cdn on public versions of the codebase. Should remedy that issue, but possibly make it harder to test.

[dereksmart]

🚀

[jeherve]

Looking good! Let's test it for real now! 🚢