Related with https://github.com/swcarpentry/styles/issues/80.

Rev 7da36a56 added a new row to the syllabus table with only 11 columns (instead of 12), which broke layout.
This fix adds the missing space.

Add label to search box and navigation arrows.

</br> instead of <br/> renders fine in the browser but not in an epub

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.