Signup events for analytics

Summary

Push signup events to analyze user conversions

• Closes #1240 (closed)
• Closes #1359 (closed)

ElasticSearch Mappings

• NEW: minds-metrics-* -> bool pro_referrer

Steps to test

• Port forward sandbox sites elasticsearch cluster to your local machine (optional)
• Open your elasticsearch browser and open minds-metrics-* index
• Sort by @timestamp descending
• Open the review site, and create a new user. Don't do anything else after that to avoid unnnecesary events.
• Refresh your minds-metrics-* view on ElasticSearch browser
• Ensure there's a signup metric with a user_guid and a cookie_id value set (might be the third or fourth row from the top). Filtering by cookie_id (which has the same value a mwa cookie on the website).
• Log out from the site.
• Create a new user using this link: https://goal-signup-cookie-id.minds.io/register?referrer=0xEMI
• Refresh your minds-metrics-* view on ElasticSearch browser
• Ensure there's a new signup metric with referrer_guid and pro_referrer set to true.

Note: This one might be tricky for @xander-miller to test as it's a backend-only storage change. The main concern is to ensure new user registration works OK.

Example data:

Estimated Regression Scope

New block of code executed at register/complete event, might affect user sign-up.