Tiled Gallery: No more carousel when it is off.

Carousel is no longer shown for Tiled Galleries when clicking a gallery item, when it is turned off in the Jetpack settings. Addressess https://github.com/Automattic/jetpack/issues/516.

To accomplish this, we simply need to make the gallery container have the data-carousel-extra attribute only if carousel is found in the Jetpack active modules, since [this](https://github.com/Automattic/jetpack/blob/master/modules/carousel/jetpack-carousel.js#L430) is how carousel identifies proper carousel containers, used [here](https://github.com/Automattic/jetpack/blob/master/modules/carousel/jetpack-carousel.js#L1366).