Load Mixpanel dynamically, remove it from index.html

[ovidiuch]

This will lead to an issue to abstract the tracking and make Mixpanel optional, maybe integrate with GA with this opportunity

Made the minimum, the tracking method is now agnostic and the Mixpanel config is using a settings var, which makes it optional. Next steps if needed:

• Create a separate script for dynamically injecting Mixpanel script tag
• Integrate with GA