Restrict updates to metadata access field

[jjnesbitt]

Closes #1787
Closes #1831

It seems both these issues were due to us not prohibiting the access field from being updated. A user should never update the access field (as it is derived from the embargo status of the dandiset), nor is there a natural way for that to happen using the meditor. However, it was not actually disallowed in anyway, so the following situations were possible:

1. After the dandiset is embargoed, if the meditor uses old metadata in some way, it can overwrite this field with the old embargoed access field. All without the user being aware, since that field isn't available in the meditor
2. If someone were to simply use the API outside of the meditor, they could set that field to whatever they want, intentional or not.

This PR updates that field to treat it as computed and thus not able to be changed from a simple metadata update. (See below)

[waxlamp]

I had two questions, but if the answers are what I think, they don't block approval and merge of this PR:

1. The new test that checks for non-mutability of the access field relies solely on the addition of access to the computed_fields list in Version.strip_metadata(), right?
2. There is no added test to see whether the correct access field value pops out of an embargoed Dandiset--is that already covered by existing tests (i.e., those tests unaffected by adding access as an immutable field)?

As I said, aside from those questions... LGTM.

[jjnesbitt]

This PR has changed slightly, as I realized that there are sometimes other sub-fields in access, that may indeed require updating (contactPoint is one such example). As such, I've updated this change to instead restrict updates to access to be in the correct format, while still allowing extra fields to be present. So access is not really being treated as "computed", rather is just handled in a special way.

Changed implementation

[mvandenburgh]

Just one improvement that I think can be made to a comment, otherwise this makes sense and lgtm

[dandibot]

🚀 PR was released in v0.3.86 🚀