Add linkable support for firefly://data URIs

We've been using/encouraging a pattern for NFT URIs that follows a URI scheme of `firefly://data/` to identify that the NFT metadata is contained in a private FireFly data item.

Assuming we do want to commit to this as a generally supported pattern, we could add link support in the UI (also assuming that token URIs continue to [show up in more places](https://github.com/hyperledger/firefly-ui/issues/201)), so that these links can be clicked to take you to the data item.