Date precision and false positives in structural diff

[strogonoff]

Here a source item specifies date as a year, edited data specifies item as ISO date, and as a result diff view is confusing:

• Dates that specify year come from legacy data
• We mandate full ISO date
• It’s unclear what to do with dates that specify only year.

To properly address the issue we should use consistent date format:

• Upconverting them to ISO date. Seems potentially misleading (if it’s unknown on which day in that year the item was actually accepted).
• Change date format in data spec format to include both a full date and then a precision value that indicates whether month/day matter.

In meantime I want to try fixing structural diff view to show that format has changed (i.e., show original item data without date conversion that happens apparently at some stage).

cc @ribose-jeffreylau