README: Update 'r-gaia-cs' -> 'rgaiacs'

I thought the hyphenated form was current when I used it in 0d8da61e
(README.md: Add local maintainer listing, 2015-04-15, #2), but GitHub
no longer recognizes the hyphenated form.  The un-hyphenated form
seems to date back to 2012-03-06 [1], so maybe I was just confused ;).
Anyhow, fixing to use the current username.

[1]: https://github.com/rgaiacs

Update JavaScript locations to avoid errors

README needs to be created from scratch.

Instructions on changing the URL linked to the "Source" button

Fix a typo in README.md

Removed lingering references to motivation.md and motivation.html

Removing Legend from required headings

Removing motivational slideshow

Removing support for motivational slideshows

Merge branch 'removing-motivational-slideshow' of github.com:gvwilson/lesson-template into removing-motivational-slideshow

Avoid wrong error with links at index.md

Clarifying challenge format

Modifying page template so that main title links to home page.

If there is a subtitle, the main title links to the `index.html` home page.  If there is no subtitle, this *is* the home page, so there is no link.

Explaining why we use Markdown

We don't validate links to glossary
but we validate links from index.md
to ensure that their text
and the title of the lesson it point to
are the same.

This exclude links point to one address with '#'
since it probably points to a glossary entry.

Update in the list of contributors from the Git history.

Update in the list of contributors from the Git history.