Merge branch 'update-template-no-css' of github.com:fmichonneau/lesson-template into update-template-no-css

add new line after list

this PR replaces #145 which also included the changes to the CSS.

otherwise, during the html conversion the ul and li tags are not placed where
they need to go.

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