Use BEM scheme in our CSS

[ry]

http://getbem.com/introduction/
https://www.joeforshaw.com/blog/css-the-bad-bits-and-how-to-avoid-them

cc @yungbeto

Since there's a lot to convert, it probably makes sense to to it one page at a time.

• Add a comment to top of main.scss explaining the organization scheme we're using. Summarize BEM in a few sentences.
• index page
• docs
• notebook index
• notebooks themselves
• references
• remove -webkit- prefixed css things. unnecessary.