Move organization_users to gitlab_main_cell

What does this MR do and why?

Move organization_users to gitlab_main_cell

• we need to have this table in each cell db
• no rows are currently in the table: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/24954/commands/79215 and we are still just using one database for now.
• examples of changing to clusterwide: !120605 (merged), !124387 (merged)
• examples of changing db in general: !130038 (merged)
• see #436015 (closed)

Note: Loose foreign keys generated with this script defined here: https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html#the-scriptsdecompositiongenerate-loose-foreign-key

Changelog: changed

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Related to #436015 (closed)