WIP: Use a new table for user contribution stats

Open username-removed-714523 requested to merge leibo/gitlab-ce:user-contribution-table into master

What does this MR do?

It defers the user contribution stats to a new table, instead of the events table.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This will allow down sizing the events table.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

#22623 (moved)