Fix: add permissions for custom fields with migration

[shamoon]

Proposed change

I forgot to include this in #4502 but as we did for consumption templates I think we should automatically add (global) permissions for custom field objects upon migration. Especially since not having these perms will cause issues on docs that have fields attached. I edited the migration rather than a new one since we havent released that yet, obv. Welcome any thoughts.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please explain):

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have included testing coverage for new code in this PR, for backend and / or front-end changes.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

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