Cells: Iteration 2: Make `users` to be `gitlab_main_clusterwide`

Problem

Make users to be part of gitlab_main_clusterwide.

Make code to:

• make the gitlab_main_clusterwide to inherit all from gitlab_main
• trigger violations (cross-join, cross-DB) only between gitlab_main_clusterwide and gitlab_main_cell
• do not trigger violations between gitlab_main and gitlab_main_clusterwide

To do this:

• we will need to implement schema inheritance, and resolving of inherited schema changes to lib/gitlab/database.rb

References

• Draft: Cells: Move `users` and `namespaces` in ... (!108462 - closed) such is implemented in here