Components: only register installables for active components

Otherwise the installables app will attempt to use the registered class,
which won't be loaded because the component isn't active.

See #404

src/classes/components.php

@@ -272,7 +272,10 @@ public function register( $args ) {
 
 		$this->registered[ $component['slug'] ] = array_intersect_key( $component, $defaults );
 
-		if ( isset( $component['installable'] ) ) {
+		if (
+			isset( $component['installable'] )
+			&& $this->is_active( $component['slug'] )
+		) {
 
 			/** @var WordPoints_Installables_App $installables */
 			$installables = wordpoints_apps()->get_sub_app( 'installables' );