Save `ipfsUrl` as a field on the Document model

[allanlasser]

The metadata field is great way to flexibly test new metadata, but by now IPFS is a pretty core part of the DocumentCloud data model, with ~1.6M documents having an IPFS URL. Alongside source and author, we can make IPFS a first-class property of documents.

[mitchelljkotler]

What are we looking to gain by making this a dedicated field? I don't think there is anything inherently wrong with a single metadata key appearing on a large number of documents