Add reading depth to popups schema based on discussion in {T145388}, such that an event is logged at set intervals while a reader explores a page.
From: https://phabricator.wikimedia.org/T145388#2661200
See https://meta.wikimedia.org/wiki/Schema:TestSearchSatisfaction2
Add a new action type of event and field to Schema:Popups to track time on page, with analogous values to the `checkin` field on `Schema:TestSearchSatisfaction2`
> checkin
> type "integer"
> required false
> description "A numeric value representing the number of seconds a user has spent on a page. As with Schema:TestSearchSatisfaction2, the pings are at 10s, 20s, ..., 50s, 60s, 90s, 120s (2min), 150s, 180s (3min), 210s, 240s (4min), 300s (5min), 360s (6min), 420s (7min)."
And also add a variable for the visibility of the page (https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API#document.visibilityState_Read_only): visible or hidden for supported browsers, null or undefined for not supported ones.
Use the existing sampling rate for Hovercards.