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

Updating style files

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.