Initial mixpanel implementation

[cjhendrix]

We may want to split this issue into multiple issues, but for now:

• [ ] configure prod/dev/localhost instances to deliver data to different mixpanel projects https://mixpanel.com/blog/2012/09/12/best-practices-updated/
• [ ] (later) flag users to ignore: https://mixpanel.com/help/questions/articles/can-i-exclude-myself-from-mixpanel-data
• [ ] Encrypt comms to mixpanel with .set_config : https://mixpanel.com/help/reference/javascript-full-api-reference
• [x] instrument interface-based resource downloads with a resource download event
• [x] instrument direct downloads with a resource download event
• [x] instrument searches from the interface with a search event
• [ ] instrument searches from the api with a search event

The events and their properties to be instrumented are marked as "a - high pty" in this doc: https://docs.google.com/spreadsheets/d/18_ExT6h2OiK6r1PYkQ6n3BEdQ1WYCA4zL28CyjDkXe8/edit#gid=0

More resource and info are in this doc: https://docs.google.com/document/d/1gIY_LXhWG2ams5siwXgrDfaWEPShgWm11W_VJ6cRw4k/edit#

[danmihaila]

moved to next sprint. @alexandru-m-g pls update what is done in Sprint 90 and what you are going to do in 91