tools: Add trailing newlines to files that were missing them

This avoids things like:

  ~$ cat tools/validation_helpers.py
  ...
          return self.is_block(ast_node) and has_heading~$

with your next prompt way over to the right ;).

CONTRIBUTING.md: Use a generic reference name for the issues link

Downstream lesson maintainers are supposed to adapt this to point at
their repository's issues [1], so use a reference name that works for
both the lesson-template and downstream repositories.  That way they
only have to update the referenced URL and can leave the reference
name alone.

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-78310923

Bring in style file changes

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

Adding contributor covenant

Fix: styles for tables in SQL lessons

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.