Updating style files

Add cards class

Add documentation about card class

Render specific blockquotes as Bootstrap panels

Better style for challenges

Fix background for lessons

There's a note in
https://github.com/swcarpentry/lesson-template/pull/182
explaining why we need to fix style for panel heading.

This makes some things look nicer (e.g. adds gradients, etc).

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

Update CSS to go with the SWC site template update

Merging core into gh-pages

Adding contributor covenant

Fixes to lesson layout instructions

there. >_< )

Changed the reference to the lesson's README file -- it's not "this file" any more (this file is LAYOUT.md). 

Also removed the ">"s from the beginning of the block of text to be included in the README, to enable copying-and-pasting (and because the ~~~ should offset the quote by itself. I hope.

Fix: styles for tables in SQL lessons

Load bootstrap from CDN