feat: update user_id column to reflect counting each anonymous visit as a unique session

What does this MR do and why?

Per https://gitlab.com/gitlab-org/analytics-section/product-analytics/gl-application-sdk-browser/-/issues/38+, we standardize on the user_id column, which will be sourced differently depending on whether users have given consent to be tracked or not.

This updates the Cube schemas to reflect this change, i.e., each page view by an anonymous user will be counted as a new session.

Sessions.js has been intentionally unchanged as it's being re-written in Add updated sessions and returning users cubes ... (!92 - merged) • Max Woolf • 16.6

How to set up and validate locally

Restart your Cube container and verify that multiple page view events are reflected as multiple individual unique users, even if they're the same person.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Closes Update cube schemas to use user_id consistently (#29 - closed)