Draft: Add namespace_monthly_usage_id column to ProjectMonthlyUsage

What does this MR do and why?

This MR adds a simple bigint column to ci_project_monthly_usages so we can tie the usage of individual projects to the namespace responsible for their consumable minutes. By attaching project-level consumption records to namespace-level consumption records, we'll be able to maintain an account of minutes used by a project when it is moved from one namespace to another.

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