Rendering and CSS fixes for code blocks.

[benwarfield-usds]

Removed two post-processing steps and added some CSS to make code blocks work again.

• the manual processing of triple-backtick blocks was a no-op, because that function is handled by the markdown2 "fenced-code-blocks" extra
• the manual removal of <p></p> tags was producing invalid HTML that was harder to format: deleted it
• the bulk substitution of newlines with <br> tags was replaced with a CSS rule
• CSS font-family rules were tweaked in order to allow the font-family rule for <code> in user-agent stylesheets to work correctly