Add stats block with shareable image generation

.github/changelog/3126-from-description

@@ -0,0 +1,4 @@
+Significance: minor
+Type: added
+
+Add a stats block that displays annual Fediverse statistics as a card on the site and as a shareable image on the Fediverse, with automatic color and font adoption from the site's theme.

activitypub.php

@@ -55,6 +55,7 @@ function rest_init() {
 	( new Rest\Admin\Actions_Controller() )->register_routes();
 	( new Rest\Admin\Statistics_Controller() )->register_routes();
 	( new Rest\Application_Controller() )->register_routes();
+	( new Rest\Stats_Image_Controller() )->register_routes();
 	( new Rest\Collections_Controller() )->register_routes();
 	( new Rest\Comments_Controller() )->register_routes();
 	( new Rest\Followers_Controller() )->register_routes();