Subtle border around non-text assets in viewer

Addresses #520
Merge branch 'develop' into 520-viewer_asset_border
Merge branch 'develop' into 520-viewer_asset_border
Merge branch 'develop' into 520-viewer_asset_border
Resize figure to wrap iframe

Previously, the figure was only resized to fit images, but I noticed that
it looked a bit odd with video content as well. YouTube seems to be the
most common video provider, and it uses IFRAMEs, so I modified the
figure-resizing code to size the figure captions and borders to wrap
IFRAMEs as well.

Addresses #520.
Updated the changelog to reflect #520

Closes #520