Performance: only load concatenated CSS when needed.

See #1258

We currently enqueue a `jetpack.css` concatenated CSS file as soon as your site is connected to WordPress.com.
That jetpack.css file is built off a list of stylesheets from individual modules that are not enqueued when the concatenated file is enqueued.

In this commit, we add a new condition: for the file to be enqueued, there has to be at least 2 modules that need it. This will avoid loading the file when it wouldn't even be needed at all.
This requires a change in the implementation: we now check for active modules before we dequeue individual assets / enqueue jetpack.css. This is an additional call to get an option (( new Modules() )->get_active()).
This also means that the new $modules_with_concatenated_css variable will need to be kept updated.

Note that this is only a partial improvement. We still need to address the larger questions around the implementation in today's world, as discussed here:
#1258 (comment)

projects/plugins/jetpack/changelog/update-concatenated-css-more-flexible

@@ -0,0 +1,4 @@
+Significance: patch
+Type: enhancement
+
+CSS Concatenation: avoid optimizing CSS loading when less than 2 modules that require it are active.

projects/plugins/jetpack/class.jetpack.php

@@ -77,10 +77,35 @@ class Jetpack {
 	 */
 	public $xmlrpc_server = null;
 
+	/**
+	 * List of Jetpack modules that have CSS that gets concatenated into jetpack.css.
+	 *
+	 * See $concatenated_style_handles for the list of handles,
+	 * and the implode_frontend_css method for more details.
+	 *
+	 * When updating this list, make sure to update $concatenated_style_handles as well.
+	 *
+	 * @var array List of Jetpack modules.
+	 */
+	public $modules_with_concatenated_css = array(
+		'carousel',
+		'contact-form',
+		'infinite-scroll',
+		'likes',
+		'related-posts',
+		'sharedaddy',
+		'shortcodes',
+		'subscriptions',
+		'tiled-gallery',
+		'widgets',
+	);
+
 	/**
 	 * The handles of styles that are concatenated into jetpack.css.
 	 *
-	 * When making changes to that list, you must also update concat_list in tools/webpack.config.css.js.
+	 * When making changes to that list,
+	 * you must also update concat_list in tools/webpack.config.css.js,
+	 * and to $modules_with_concatenated_css if necessary.
 	 *
 	 * @var array The handles of styles that are concatenated into jetpack.css.
 	 */
@@ -6231,6 +6256,18 @@ public function implode_frontend_css( $travis_test = false ) {
 			$do_implode = false;
 		}
 
+		/*
+		 * Only proceed if at least 2 modules with concatenated CSS are active.
+		 * There is no point in serving a big concatenated CSS file
+		 * if there are no features (or only one) that actually need some CSS loaded.
+		 */
+		$active_modules                = self::get_active_modules();
+		$modules_with_concatenated_css = $this->modules_with_concatenated_css;
+		$active_module_with_css_count  = count( array_intersect( $active_modules, $modules_with_concatenated_css ) );
+		if ( $active_module_with_css_count < 2 ) {
+			$do_implode = false;
+		}
+
 		/**
 		 * Allow CSS to be concatenated into a single jetpack.css file.
 		 *