Versions: allow editable versions

[agjohnson]

We have talked about this work in a few places, this is an issue to track but feel free to fill in the blanks here.

There is some initial exploration of the code changes here:

• Allow users to edit Version's identifier and/or slug #11814

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• Continue exploring code changes and testing
• Start thorough testing to find edge cases where override fields cause problems -- syncing versions

From here we can decide what else needs to be done to support all of this.