Implement Optimizely snippet

Segment can't insert this dynamically for us since Optimizely wants to
modify the DOM.
Actually, Optimizely recommends loading way early
Don't use Optimizely when DNT is on
Merge pull request #2857 from gratipay/optimizely

Implement Optimizely snippet
Removing Segment and switching back to pure Google Analytics
Remove commented out code

Let's kill this while we're at it. DEBUG was used by old versions of
oauthlib.