Handle ProjectNamespaces when transfering a project or a group

What does this MR do and why?

Keeps a project's associated project_namespace record in sync on path, name, parent and visibility_level with project's changes.

This does not handle routes changes yet. That is going to be handled as part of #337103 (closed) which currently has a couple POCs in works: !69702 (closed), !70769 (closed), !67664 (closed)

re #340491 (closed)

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Related to #340491 (closed)