consolidate badge classes to swc.css

Thanks to ocefpaf for request that.

Add variable to represent Pandoc

Pandoc had some changes in the last versions.
Having a variable for Pandoc will help
maintainers to handle different versions of Pandoc.

Figures should be centered
and not overflow the screen.

Add article tag as wrap for lesson content

CONTRIBUTING.md: Remove the maintainer references

README.md: Add local maintainer listing

This is the new plan [1], instead of referring to a central listing
from CONTRIBUTING.md.

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-89643048

Because we're going to be storing maintainer information locally in
the README.md [1].

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-89643048

Remove logger name (root) from console messages

Ignore conduct.md and contributing.md. No, really.

(even though the contents are not validated)

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 ;).

Updating docs to reflect separation of repositories.

1.  Explaining how to use GitHub import instead of creating a repository manually.
2.  Explaining the difference between `lesson-template` and `lesson-example`.

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

the class removed are unused as we take advantage of classes provided by
Bootstrap instead.

Updating style files