Refactor StandardContext to use ids instead of objects

What does this MR do and why?

As a preparation for implementing Internal Events Tracking this refactoring makes is possible to create a StandardContext object with a namespace_id, plan, user id and/or project id instead of a Namespace, User and/or Project object.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #407858 (closed)