Replace filename with caption/description

* when the caption is available, use it as the image title
* otherwise, fall back to description if available
* otherwise, fall back to filename
* if both caption and description are available, show caption as
  title and show description below the fold; otherwise don't show anything
  below the fold.

Change-Id: I9738ca46620d7cf3b30f6e46790a37966d4b9eaf
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/589