Full Site Editing: account for -wpcom suffixes on dotorg installs

[mattwiebe]

Themes installed from wpcom on dotorg installs will receive a -wpcom suffix and fail this check. Now they won't.

Testing instructions

1. Get this branch installed and activated as a plugin on a dotorg site.
2. Download Modern Business to your dotorg site. direct link
3. Activate Modern Business in wp-admin
4. In a wp shell session on your dotorg site, verify that the following returns true:

\A8C\FSE\Full_Site_Editing::get_instance()->is_supported_theme( get_stylesheet() )

This is needed for Automattic/jetpack#13196

[matticbot]

Test live: https://calypso.live/?branch=fix/fse-is-supported-theme-in-jetpack

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[noahtallen]

Interestingly we also rely on normalize theme slug here:

wp-calypso/apps/full-site-editing/full-site-editing-plugin/full-site-editing/templates/class-wp-template.php

Lines 60 to 66 in 4e996aa

	public function normalize_theme_slug( $theme_slug ) {
	if ( 'pub/' === substr( $theme_slug, 0, 4 ) ) {
	$theme_slug = str_replace( 'pub/', '', $theme_slug );
	}
	return $theme_slug;
	}

We just copied the function over manually because... Iirc the context for FullSiteEditing was different from the context of WpTemplate which made it pull the wrong theme slug. I.e. it pulled the theme slug of the API wordpress instance instead of the actual site instance.

Could just add these lines there, but it may be worth putting the function somewhere which can be used by both classes.

[Copons]

I wonder, from a purely performance standpoint, if it would be better to use a substr( $theme_slug, 0, -6 ) instead of the regexp, considering we know both the string length and position.
(Same could be said for the previous str_replace, btw)

[mattwiebe]

Could just add these lines there, but it may be worth putting the function somewhere which can be used by both classes.

It seems like a fine idea but, as these plugins may be split out in some future, we'll just leave 'em separate. If these ideas.

I'll update both functions for now, using @Copons' optimization suggestion, too.

[Copons]

LGTM!