add caption and owner to asset helper, add CSS to style captions

A basic implementation of captions/credits on article assets:

* Updates asset helper to include caption and owner
* To conserve real estate on small screens and not distract from the reading experience, a caption invitation is overlaid in the bottom-right corner of the asset and can be revealed on hover.
* On screens above 500px, captions display on the asset by default (no affordance or need to hover to reveal caption)