Load Oversegmentation Meshes during Proofreading

[fm3]

Second try with cherry-picked commits from #6517

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• abc

Issues:

• fixes #6506

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• [ ] Updated (unreleased) changelog
• [x] Needs datastore update after deployment
• [ ] Ready for review