Make import_guests command more lenient on source

The database constraint on the Identity model demands only one value per
id type for each person. This means that a person with a pass number
already in greg from a different source than the one we are trying to
import will trigger an integrity error when we try to create it.

The logic now instead checks for identities with other sources as well,
and if a match is found from another source system, the source is
updated to the one in the data we are importing. This will also mean
that a previously entered manual entry, will be converted to one marked
as automatic.