Small tweaks and fixes to the styled-text module

Dokku: https://styled-text.dokku.hypothes.is

 * Move the styled-text module into it's own file.
 * Add markdown examples to the pattern primer.
 * Fix list item, blockquote and code styles.
 * Improve font sizing of heading elements.
 * Ensure code, serif and sans-serif are rendered at similar sizes.
 * Style sub lists.