Fix "br" typo causing ugly rendering (epub only)

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

fix swc_github_io versus swc_github

index.html uses `swc_github_io`, _config.yml uses `swc_github`. This is I think what caused links in the final workshop website to be wrong.

Updated Git install instructions

changed link to windows installer under SQLite

An internally-defined Markdown link has the form [xxx][yyy].  If the
ID 'yyy' doesn't resolve, the text is left as-is, so we check for
that, then subtract those that reference configuration values using
'{{'.  It's a hack, but it'll catch at least a few things.

Import future print in case of python2 environment

[fix # 82] Check R packages required by the lesson

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`.

Fixing citation