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.
add Other Optional Stuff section, link swap guide
omit needless words
Always use lower case URLs, fix date formatting and add full date title
Lowercase md file
Merge pull request #2076 from riking/clean-site-settings

Clean up site_settings.yml
BUGFIX: overflow hidden messes stuff up for chrome on mac
update onebox gem
FEATURE: Live update edits
FEATURE: update likes and flags live.
oops
Live update poll votes.
Merge pull request #2166 from vikhyat/poll-plugin

Live update poll votes
FEATURE: refactor notice code, add optional global notice.

Set the global_notice site setting to notify users of a critical event.
FEATURE: on initial boot hint users on how to get admin
improve copy
meta "See this guide to learn more" links
minor login install hint copyedit
Remove documentation references to "pg_dumps/development-image.sql".

There is no such file. Seeding is accomplised via the rake tasks as of
9ab743f351e612830a27afaab8e9b3ddd59da9e2.