Probably because of change on GitHub API.

Close swcarpentry/styles#246

Based on https://github.com/swcarpentry/lesson-example/pull/144/files

Close #178

Using `site.github.url` to refer to the repository fails to load resources when the GitHub Pages site is accessed via HTTPS, because `site.github.url` is `http` protocol.  This modifies loads of CSS and Javascript files to always use `https`.

1.  Use site.root for root path if set, otherwise use site.github.url.
2.  Display extra files for workshop-template repo as well as for lessons.