Removing FIXME from instructors.md

Sync gh-pages with newest core changes

Fix validation of motivation.md

Revert changes when validating glossary

Skip mailto links in link validation

Removing unused files. Thanks @gvwilson, it looks good.

Merging CSS from styles repository

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

add missing bracket

Upgrade bootstrap

Add validation for callout boxes

Implement template-specific callout rules. Add tests and fix bugs revealed.

Narrow def of callout: block must have heading.

Simplify way that callout counts are tracked, improve log messages, and add further unit tests of callout validation.

Ensure that callout validation is actually run

Conflicts:
	css/swc-bootstrap.css
	css/swc.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.

Join CSS from site and lessons

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

-   Remove extra space before title of boxes
-   Remove background color of boxes when printing
-   Add "Input", "Output" and "Error" tips for code blocks
    when printing
-   Avoid duplicate lines

and "learning objectives" box.