Carousel: add 'srcset' and 'sizes' support to images loaded in Carousel view

### Impacted plugin

Jetpack - Carousel module

### What

When you open a photo with Carousel, it loads the full resolution size. For better performance, it would be nice if it used 'srcset' and 'sizes' to load a more optimized image size. The "View Full Size" link is still there at the bottom for people who want to view the full size.

Start by locating the Jetpack Carousel module's image-loading entry point and reviewing how Carousel currently opens photos. Confirm the optimized image uses srcset and sizes while the “View Full Size” link still loads the full-resolution image; validate the behavior in the Carousel view.

Written by the indexing model from the issue text.