Organization contains namespaces

What does this MR do and why?

Extend the Organization model to contain namespaces and groups. The relationship will default associate to the default organization. The default serves two purposes:

1. Set the column value without having to perform a slow backfill operation.
2. Delay having to write code to explicitly reference namespace organizations.

We constrain the relationship between the organizations and namespaces table using a loose foreign key.

I've created a follow up issue to index namespaces.organization_id synchronously.

More information around cells and Organization can be found at https://docs.gitlab.com/ee/architecture/blueprints/cells/cells-feature-organizations.html

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Closes #394796 (closed)