Fix "Enter" issue, hide scrollbar, compress with UglifyJS 3

[fabiomus]

Hello, this pull request fixes the "Enter" issue. When the lightbox was open, each time the Enter key was pressed, another lightbox would open on top of the others and this would lead to a lot of open lightboxes and the need to close them one by one to get back to the page. It also hides the scrollbar while the lightbox is open and shows it again when it closes. The code has then been compressed with UglifyJS 3 online. I decided, for the sake of consistency, to use "===" instead of "==" and single quotes instead of double quotes for strings.

Hope this is helpful. Fabio