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.

"join" filter only works on lists (arrays), so when `instructor`
or `helper` are strings (or empty objects) then the result will be "".

But when they are arrays of strings, then the result will be like:
"Piotr Banaszkiewicz|Greg Wilson".

People can be named "John Smith, Jr.", but (probably) not
"John Smith| Jr."

Close #183.

the banner needs to be outside the "container" to be in the right corner

2. Creating a simplified generic page template for other workshop pages.