Render specific blockquotes as Bootstrap panels

From http://getbootstrap.com/components/#panels-alternatives
the only panel that the font color is white is
`panel-primary` and for that reason it is the only one that we can't use.

This replace the use of `panel-primary` with `panel-warning`
since prerequisites and learning objectives **never**
appear in the same file.

Blockquotes that have a header (with class "callout", "challenge",
"prereq", "objectives") as their first element will get rendered as
<div>s.  Their structure will resemble Bootstrap's panel:
http://getbootstrap.com/components/#panels-alternatives

There's currently an issue with pandoc: the header nested under the
blockquote will never have assigned attributes.  This is causing the
panel's heading to look enormous.  I'll fix it in
https://github.com/swcarpentry/styles/pull/11.

BTW: glyphicons won't render correctly unless 'fonts' directory gets
moved up one level to the 'css' directory. I'll fix that in
https://github.com/swcarpentry/styles/pull/11 too.

Core and gh-pages aren't sync

This is based on 966a0322 from gh-pages.

Thanks to @ChristinaLK to report the problem.

Validator should ignore CONDUCT.md and CONTRIBUTING.md

Adding contributor covenant

Add more lables to to setup labels script

Do not render CONTRIBUTING.html. Ignore it if it is built.

Fix link to Pro Git chapter on contributing to a project.

A generic CONTRIBUTING file

Minor fixes to license file.

1.  Removed YAML header.
2.  Changed link to refer to `LICENSE.html` instead of `LICENSE.md`.

Leaving the reference to the lesson-template repo alone - lesson authors will be expected to edit this (and instructions to do that will go in the docs in the gh-pages branch).

Validator should fail when FIXME present

Add validator target to makefile

Update validator to work with revised LICENSE.md.

1.  Remove copyright information about code of conduct (which isn't in this repo).
2.  Change ownership of trademark to NumFOCUS.
3.  Use uniform link syntax.

First should come the common workflow tasks, then commands for internal/debugging use.

Our authors use this label to mark unfinished/work in progress lessons.

Fix validation of motivation.md

Skip mailto links in link validation

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