Minimize clusterwide tables
## Problem statement
Cluster-wide tables need special handling, and clusterwide [syncing](https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/cells/decisions/014_clusterwide_syncing_in_cells_1_0/) is out of scope for Cells 1.0
The tables marked as [cluster wide](https://docs.gitlab.com/ee/development/database/multiple_databases.html#gitlab-schema)
should be re-organized as Organizational features. There should no longer be any new cluster-wide features.
## Exit Criteria
- [x] User related tables moved from `gitlab_main_clusterwide` to `gitlab_main_user`.
- [x] All remaining `gitlab_main_clusterwide` tables moved back to `gitlab_main`. These tables can then be moved to any other schema listed in https://docs.gitlab.com/development/cells/#available-cells--organization-schemas.
- [x] No more `gitlab_main_clusterwide` tables are allowed to be added.
## DRI
TBD
## Next step
- [x] Look at all 30 odd `gitlab_main_clusterwide` tables to determine which needs to be synced to all tables, or not. => https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/cells/decisions/014_clusterwide_syncing_in_cells_1_0/
- [x] Convert cluster settings tables to `gitlab_main_clusterwide_settings`
- [ ] Move user related tables to `gitlab_main_user`
- [x] Develop plan to minimize / stop new cluster-wide tables.
epic