Add SimpleLightbox 2.13.0

This turns the Portfolio page into a lightbox: clicking an image will open its enlarged version, and users can navigate through images. Version 2.13.0 allows swiping in addition to back/forward buttons.

This is essentially a backport from the original Start Bootstrap theme at https://github.com/StartBootstrap/startbootstrap-creative, which added this in 71a04770.

In addition to backporting this functionality, I upgraded SimpleLightbox from 2.1.0 to 2.13.0 and added full-size versions of the stock images to better illustrate the functionality.

The originals of the images used in the Portfolio section were taken from https://pxhere.com/ and are under CC-0, thus they should be OK to use here.

For a demo, see https://mvglasow.gitlab.io/creative/