This adds an image feed that you can scroll through with hotkeys
in the lightbox.
The left and right arrow keys along with the left and right arrows
will go to the prev/next image, and clicking on an image will also
take a user to that image.
This consolidates all actions to close modals into modals.js and
triggers the correct cleaning/collapsing function dependent on what the
data-overlay attribute is labeled as.
It also ensures these all have an e.stopPropagation().
Fixes#4029.
