(Bug 48622) When unserializing Snaks, consider faulty data values JSON

If unserializing a data value fails, a UnUnserializableValue DataValue will be created in the place
of the actual data value. This could for example happen if the expected JSON changes for some
reason and no script ran over existing data to migrate existing JSON to the new representation.

NOTE: This depends on I4dba219 in the DataValues extensions repository

Change-Id: I4f9215add1f3e964f229cbe222e9b0562045648d
Merge "(Bug 48622) When unserializing Snaks, consider faulty data values JSON"