Note Mixpanel in Privacy Policy; #686
Add dev Mixpanel token to default_*.env; #686
Implement a mixpanel funnel; #686

This funnel has three events: Opt In, Explicitly Tip, and Add Credit
Card. The first is fired server-side immediately after an alias is
performed (see comments inline). The other two are fired client-side,
and any new events should be fired client-side as well.
Bump version to 7.4.9
Bump version to 7.4.9-dev