Set descriptive User Agent string for IATI Registry interactions

[robredpath]

We're trying to understand usage of the IATI Registry API better, and so we're updating all IATI Secretariat tools to have descriptive UA strings so that we can track their usage of the Registry API.

Please can you update IATI Publisher to have a UA string along the lines of "IATI Publisher" so that we can track it?

cc @emmajclegg