Import changes to sanitize.py from stagestosaturn.git

 * Add insertBefore and insertAfter helpers
 * Add pad helper
 * Add replaceWithChildren helper
 * Reverse the order of empty element removal
 * Extend the 'p' whitespace cleanup to more elements
 * Consider ` in quotify
 * Quotify more elements
 * Extend the element whitelist
 * Whitespace changes

These changes have been verified to only result in whitespace diffs.