Clean up site_settings.yml

 - Hide several variables that the client does not use
 - Use concise default syntax when possible
Shuffle around the order of site settings

The goal of this is twofold:
 - Make the more commonly changed settings higher
 - Find groups for more uncategorized settings

Additionally, the SEO category was deleted, its contents folded into Security and Spam.
Merge pull request #2076 from riking/clean-site-settings

Clean up site_settings.yml