When performing migrations, and need to remap external users to local
ones, when no local mapping is possible, map the external user to Ghost,
rather than the user who initiated the migration.
Mapping the external user to the migration initiator has the potential
of breaking assumptions elsewhere, like only having one review per pull
request per user. Mapping these migrated, locally unavailable users to
Ghost makes sure these - often hidden - assumptions do not break.
Fixes #3860.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
