As per swcarpentry/lesson-template#140, but only the core CSS
changes.

Improve script to setup labels

Validator should check that required files exist

Use unified CSS from site.

As per swcarpentry/lesson-template#140, but with only the HTML changes

As per swcarpentry/lesson-template#140, but only the core CSS
changes.

- Delete GitHub's default label that will not be used
- Create new labels
- Update color for labels

Add debugging notes about Git and line endings

Import style for slides from https://github.com/swcarpentry/slideshows

Excluding unwanted files from build

Update LAYOUT for changes from #103

Change filters since classes must be explicit

Add examples of links to part of our lessons

Add classes

All classes **must** be explicit because this will make easy
to translate the lessons.