Adding Opt-in Analytics (mixpanel) Support

Opt-in value to fill in `MIXPANEL_PROJECT_TOKEN` env var and send it to frontend pod.
We should keep this opt-in  - it's both an unexpected behavior, and also will most likely not be wanted or possible for most self-hosted installations (no internet access).

This is not considered secret, but can arguably be in the secret and not configmap 🤷 