Add Google Analytics Tracking

[lukasschor]

Add basic Google analytics tracking for the Safe for Teams. Users need to actively opt-in to be captured by the tracking.

Tags (request access): https://docs.google.com/document/d/19ykD6INGkFMvQccmegaFAcaj-lk4jUBJB5IAY0BkSo4/edit?usp=sharing

[pcowgill]

But require that users opt in before turning this on, please!

[lukasschor]

Of course!
Only users that explicitly opt-in to help us improve the interface will be part of the analytics tracking.