Reset permitted YAML column classes during to_prepare

What does this MR do and why?

Reset permitted YAML column classes during to_prepare

Fixes #582502 by ensuring the reloaded classes are permitted.

How to set up and validate locally

Start with your GDK on master.

1. Navigate to a commit URL, like http://gdk.test:3000/some/project/-/commit/69285ea0273cadaae3d1b0c07b256001bb1b3a87.

2 Leave a comment on the diff. Note that it works:

(If you had to run migrations, it will probably fail for the same reason as #582502, and you get to skip straight to step 4 — but if you want to retry the whole thing, restart your GDK and go again.)

3. Make a trivial change to the code so that it needs to reload. For example, add a character to a body string somewhere in app/services/system_notes/issuables_service.rb.

4. Try leaving another comment on the diff. It should fail, and the Network inspector in dev tools will show the notes?html=true call failed with the traceback shown in #582502.

5. Check this branch out and restart GDK.

6. Run steps 1 through 4 again. It should work! 🎉

MR acceptance checklist

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/cc @engwan