Add foreign key to organization_users.organization_id column

What does this MR do and why?

organization_users table links users to organizations. Originally, this was implemented using loose foreign key. But since both organization_users and organizations are now part of gitlab_main_cell, we can use database foreign keys.

In a previous MR, this already happened for user_id, this MR will also make organization_id a foreign key.

References

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• #493506 (closed)

MR acceptance checklist

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Migration output

Up:

main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE organization_users ADD CONSTRAINT fk_8471abad75 FOREIGN KEY (organization_id) REFERENCES organizations (id) ON DELETE CASCADE NOT VALID;")
main:    -> 0.0015s

Down:

main: -- transaction_open?(nil)
main:    -> 0.0098s
main: -- remove_foreign_key(:organization_users, {:column=>:organization_id})
main:    -> 0.0027s

Related to #493506 (closed)