Close #178

Close #176

Close #177

This is based on https://github.com/swcarpentry/lesson-example/pull/127/.

By doing this, when a page is heavily zoomed in (e.g. 500%) the navbar
icons stay in the same row, rather than breaking onto two different rows.

by run

~~~
$ find _includes -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find _layouts -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find _extras -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find _episodes -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find bin -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find assets -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
~~~

Close #155

Rebased of https://github.com/swcarpentry/lesson-example/pull/116/.

Thanks to @tracyteal.

Includes @drjwbaker's description of Library Carpentry.

As suggested by @gcapes.

Related with https://github.com/swcarpentry/styles/issues/80.

Jekyll doesn't export the _episodes_rmd so we need to flag the files.
Each Rmd file must have in the YAML header

    source: Rmd

https://github.com/swcarpentry/lesson-example/pull/104#issuecomment-283310978

Rev 7da36a56 added a new row to the syllabus table with only 11 columns (instead of 12), which broke layout.
This fix adds the missing space.

1.  Create file `_includes/links.md` with definitions of all links.
2.  Replace links at the bottom of Jekyll'd `.md` files with inclusion of that file.
3.  Modify `bin/lesson_check.py` to load and check definitions in this file.

This eliminates most redundancy in link definitions, though
`CONTRIBUTING.md` and `README.md` still contain their own links, since
they are not Jekyll'd.  Note that we *cannot* put the links into
`_layouts/*.html` (or even `_layouts/something.md`) because of the
order in which Jekyll expands things - we have to explicitly include
it in each page.

Add label to search box and navigation arrows.