Funnel by platform

Build a 3 step funnel per platform, counting users (not events):

1. viewed, users with at least one view_item
2. carted, users who had an add_to_cart strictly after their first view_item
3. purchased, users who had a purchase strictly after their first add_to_cart that counted in step 2

Steps have to happen in order. Adding to cart before ever viewing an item doesn't count for step 2, and buying before that qualifying add to cart doesn't count for step 3.

Use the platform from users. Only include platforms that have at least one viewer.