Improve WikiData import

[anjackson]

The basic WikiData import is working okay, and getting the basic stats right, I think(#13). But some improvements would be useful.

• Looking at http://www.digipres.org/formats/mime-types/#application/applefile we can see WikiData is aware this format has two versions, but this is not currently being mapped into a version= parameter on the MIME type. It would be fairly straightforward to use the version property from WikiData, assuming we can make the query generate it and we store it appropriately.
• Gather WikiData fields on download. Rather than the current denormalised list, it would likely be less error prone to collect the various extensions/MIME types for each Q####### ID when processing the downloaded query results for storage. This would then mean changing the aggregateWikiData function to match, but would make it much simpler.
• ...???...