Cells: Iteration 2: Convert FK between `users` and `members`

Once we do Cells: Iteration 2: Make `namespaces` to be `gi... (#390102 - closed) this will define a root table for users (located in Cluster) and members (located on Cell).

The Cells: Iteration 2: Record all queries performe... (#404762 - closed) does identify all other tables that are touched as part of User can create Group.

We need to:

• Assign each of those tables accordingly to gitlab_schema, and in case of cluster models change it's MainClusterwide::AplicationRecord base class
• Identify cross-joins similar to Cells: Iteration 2: Ignore cross-joins, cross-d... (#409840 - closed), create issues, fix one related to User can create Group
• Convert foreign keys crossing gitlab_main_clusterwide and gitlab_main_cell as done in Cells: Iteration 2: Convert all foreign keys to... (#409837 - closed)

It is likely this constraint that needs to converted:

    ADD CONSTRAINT fk_2e88fb7ce9 FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;

Proposal

Convert fk_2e88fb7ce9 constraint to a loose foreign key. That way when record is deleted on users, we cascade the delete to members still