Replace basic Images with Media in Core, Caption & Attribution in IU Paragraphs

The IU Framework suggests that Images should be accompanied with Captions and Attributions (however their example code doesn't show this in action), they are using the <figure> tag as a wrapper, which suggests that <figcaption> should also be used.

Drupal 8 now has the media module in core. We should probably switch the iu_image paragraph type to support Media entities and add fields/formatters there for captions/attributions.