Fix z-index bug in Firefox that would make navigation buttons appear on top of the metadata panel

Also fixes other issues:
- Some code in mmv.lightboxinterface.js wasn't doing anything
- Canvas buttons were being added to the wrong element
- Several CSS rules were being declared twice, a remnant of the multilightbox days

Change-Id: I6ffa1f6a989964d3863aa9dbeb332c0e59dff2e6
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/409