Allow existing entries to be updated with new values

[mmmatthew]

Must be solved before #2

Currently, data is only carried into wikidata if the entry does not already have a value for the corresponding property (exception: catalog code). However, it should be possible that if the data to import is updated, these changes can be imported automatically.

Option 1: Check who is the author of the change and allow automatic overwrite if the author is the bot
Option 2: Associate each property value with a reference, and use the reference to decide whether to update: if the reference is an older version of the WVZ dataset, then overwrite and update reference.

Option 2 is probably the better way to go. It introduces the challenge of having, for a given property, multiple values from different sources. Problem: for our web application, which value do we show?