Add UTC info to FAQ

Two people asked about it today.
Merge pull request #691 from scraperwiki/add-time-zone-to-twitter-search-faq

Add UTC info to FAQ
Remove classic CSS
Remove mention of Classic signin
Remove Classic retirment guide

It wasn't up to date anyway.
Reorganize help pages
Update overview help
Separate Twitter FAQs into own page
Redirect old #faq URLs