Updated UserObserver#update_mixpanel_for to use the new key,value structured User.mixpanel_fields
Updated UserObserver#update_mixpanel_for to use the new key,value structured User.mixpanel_fields

Factlink/factlink-core,daukantas/factlink-core,Factlink/factlink-core,Factlink/factlink-core,Factlink/factlink-core,daukantas/factlink-core,daukantas/factlink-core,daukantas/factlink-core