UI around "The related activity ID in related-activity/@ref should exist in the file being validated or in the IATI Datastore." (1000.0.6)) #976

This check is only done at the time of validation, not the time the user looks at it.

ie. It's totally possible for the activity not to exist in the datastore at the time of validation but then for processing to be finished 5 mins later and for it to then exist.