Specify release repo unambiguously

Fix SVG file for Firefox

This uses a newer version of LibreOffice to build the SVG files.

~~~
$ ls fig/*.svg
fig/file-mapping.svg  fig/forking.svg  fig/repository-links.svg
$ libreoffice --version
LibreOffice 6.0.1.1 00m0(Build:1)
$ for i in *.odg; do libreoffice --convert-to svg $i; done
~~~

Improvements over #172

Related with lesson-example/pull/172

support screen readers

Fix broken link for Contact

Add current year to copyright of workshop footer

Updated lesson_initialize

Fix CSS to use .language-X class

[fix #171] remove "clip-path" attribute

Advise users not to edit .md files generated from .Rmd files

Fix class mismatch in formatting lesson

The 'input' (raw) block showed the `.source` class identifier, but the rendered output used `.language-python`.

Ensure  Ruby is updated to work with Jekyll on OSX

If you have an out dated Ruby version. 
They may fail to work with jekyll

secure hyperlinks to pre-empt server-side HTTPS redirects

fix link to setup instructions

update maintenance process

remove episode length

indent HTML

update info on formative assessments