Add updated sessions and returning users cubes for MVs

What does this MR do and why?

• Updates the Sessions cube to read from the new Sessions table introduced in this MR. (It is a hard dependency!)
• Creates a new cute ReturningUsers to help calculate returning user sessions.
• We'll need to be careful when this is deployed to production as it relies on changes to the hard-coded dashboards too.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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

Related to: gitlab-org/gitlab#426092 (closed)