Ensure eav.register() Maintains Manager Order

[Dresdn]

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• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc.)
• I have created at least one test case for the changes I have made
• [] I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

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• Bugfix

Other Information

This addresses an issue where the order of managers in the model's _meta was not maintained correctly when adding a new EntityManager. The creation_counter of the new manager is set to ensure it maintains the correct order relative to existing managers.

I discovered the issue when attempting add a new manager, yet Django wanted to make migrations to the managers and the default manager wasn't available in the a migration RunPython call.