[IATI] activity-identifier test/check

[stevieflow]

@kindly @Bjwebb this is something I've posted as an idea on Code4IATI: codeforIATI/iati-ideas#37

If it's possible to query iati-identifier via the Datastore API, then perhaps this test could be something we could incorporate into IATI CoVE?

[stevieflow]

@andylolz has worked wonders on this via https://github.com/codeforIATI/activity-id-checker

I talked with @kindly about how we could integrate such a check into CoVE. It could be that we pull in the flat list of identifiers and undertake a kind of codelist check on this list (in the relevant elements in any IATI XML).