Plugin tweaks: Ensure jetpack blocks are available and active.

This enables the subscription widget across the network.

Follow up to 8751663
See WordPress/wporg-parent-2021#170

mu-plugins/plugin-tweaks/jetpack.php

@@ -7,8 +7,23 @@
 /**
  * Actions and filters.
  */
+add_filter( 'jetpack_get_available_modules', __NAMESPACE__ . '\available_jetpack_blocks', 100, 2 );
 add_filter( 'jetpack_active_modules', __NAMESPACE__ . '\activate_jetpack_blocks' );
 
+/**
+ * Add "blocks" module to available list, using `min_version` to ensure it's active.
+ *
+ * @param string[] $modules     Array of modules (module name => version introduced).
+ * @param string   $min_version Minimum version number required to use modules.
+ * @return string[]
+ */
+function available_jetpack_blocks( $modules, $min_version ) {
+	if ( ! isset( $modules['blocks'] ) ) {
+		$modules['blocks'] = $min_version;
+	}
+	return $modules;
+}
+
 /**
  * Ensure "blocks" module is active.
  *