Fix item sync
Fix item sync

When de-serializing item revisions from the local store,
the serialized ItemContent instances were de-serialized to
plain JavaScript objects without prototypes set.

This is the minimal fix for the issue for the top-level ItemContent objects.

The larger plan is convert ItemContent and fields to use
interfaces rather than classes so they become simple to
de(-serialize).

robertknight/passcards,robertknight/passcards,robertknight/passcards,robertknight/passcards