Split off gitlab_main_user schema

What does this MR do and why?

Split off gitlab_main_user schema.

A big part of gitlab_main_clusterwide user is user-related data, so we segment this to its own schema. For now it is planned to have user login features be clusterwide, so this change will facilitate this while preventing other user related features from escaping organization isolation more than necessary.

• gitlab_main_user is not allowed to cross-FK with gitlab_main_clusterwide
• For now, gitlab_main_user is allowed to cross-FK with gitlab_main_cell
• Additionally, gitlab_main_user must have a sharding key, only to users table.

References

• Deprecate gitlab_main_clusterwide with gitlab_m... (#522923 - closed)
• Move user-related tables to be `gitlab_main_user` (#505754 - closed)
• Add further Organization User details (gitlab-com/content-sites/handbook!13503 - merged)

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