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.