[#865] Remove IATIProjectResource.sync_owner

The IATIProjectResource.sync_owner field is causing the Cordaid IATI
import to be very slow, so we're removing the field and setting the
ownership in the post import step instead.
Merge pull request #867 from akvo/feature/865_fix_slow_cordaid_import

[#865] Remove IATIProjectResource.sync_owner