Make sure <figcaption> element has text

Make sure an asset's ``<figcaption>`` element has text before
copying it for a popup caption. Some assets may have ``<figcaption>``
elements containing things like a list of datasets, or attribution,
but no caption text.

Addresses #717
CSS classes for asset_id and asset type

Include CSS classes for the asset_id and the asset type in the
asset's container element.  This will facilitate type-specific
styling. It also makes it easier for developers to look up an
asset when viewing front-end display of information.
Hide popup captions for YouTube videos

The popup captions look weird over YouTube videos, especially
since the video embed is fixed-width.  Hide them using a CSS
style matching the ``asset-provider-youtube`` class.

Addresses #717