The great Jekyll-to-Sculpin migration

If 1.0 was the original site built with Jekyll by the good @philsturgeon, and the 2.0 was the redesign crafted by @reinink, then this must be a 2.5, since not much has changed from the outside, but the actual static site generator is now Sculpin instead.

This transition also involves the switch to Platform.sh in place of GitHub pages, and comprises a great variety of bug fixes and improvements.