HG-330 Always show caption in lightbox

This change affects lightbox media in landscape view. Currently the
caption is hidden by an image if that image is resized large enough to
cover it. Here, the z-index of the caption is increased to match the
header (which keeps the close X viewable). Consequently, the caption
area will cover a portion of the bottom of the image in this situation.
This commit also fixes a JS error when resizing images in lightbox view.
Merge pull request #347 from Wikia/HG-330

HG-330 Always show caption in lightbox