Improve annotation typography

 * 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.
Ensure styled-text is applied to all markdown docs
Fix styles of the home/help/auth pages

We now use the styled text module here and overwrite it to work with
larger copy.
Move textarea styling into form module
Merge pull request #1439 from hypothesis/styled-text

Small tweaks and fixes to the styled-text module