Display black overlay while the viewer JS is loading

I went for this option because it was the fastest to implement.
I think we should wait until we make the change to core to expose
image dimensions before we consider switching to another strategy.

Change-Id: I61c9342a2d6d6fc24a24e0988b3cf7f9a06859a2
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/248
Merge "Display black overlay while the viewer JS is loading"