Show caption on thumbnails

processThumb() is a mess. This change is kept small since we might
want to backport it per the bugzilla comments; I might follow up
with some refactoring in a separate commit.

Bug: 62518
Change-Id: I1f916b88fe1b667c6c7e51c9bdca186d4aa2ef2c
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/338
Merge "Show caption on thumbnails"