Adapt IATI Publisher to use new data registration API

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Alongside the work to change how users register and sign in to IATI products (tracked in #1486, details to be supplied when we've worked them out), we're also changing how people manage the URLs of their data on the IATI Registry. We're doing this through a new API which will sit in front of the Registry and integrate the registration/update process with the wider approach to managing information about individuals and organisations within the Secretariat.

We don't have any specific details about this yet, but we will share them as soon as we can.