Move viewer image caption to the top on hover

Addresses #717
Tweak styles for "popup" image caption in viewer

Give ``<figure>`` relative positioning, so absolutely positioned
``<figcaption>`` is at top of the figure and not its parent.

Add margin so caption follows padding around image.

Addresses #717
Always show caption at bottom of asset and popup

Instead of moving the caption element to the top of the container,
copy its contents to a separate element and always show the caption
at the bottom.

This prevents content jumping around when hovering toggles the
popup. It also makes the caption immediately visible as the user
scrolls below the top of the image.

This approach no longer allows for a pure CSS solution as we need
to use JavaScript to create a separate element.

Cleanup trailing commas in object literal definition.

Addresses #717
Add rgba test to Modernizr build

This is needed to provide classes to trigger a fallback for
semi-opaque backgrounds for popup captions.

Addresses #717
Use transparent PNG for background when no rgba support

For IE8 and other browsers that don't support rgba, use a transparent
PNG for the background of the popup caption.

Addresses #717
Merge branch '717-viewer_popup_captions' into develop
Update changelog for #717