Handle updates better in guest import

Introduces the --update-existing option, set to False by default. Thie option controls whether or not we want to update already imported information with the info from the file.

This is usually not what we want since sponsors may have changed the end dates for already imported roles after the previous import. We choose to leave it as an option so that we CAN overwrite information if we really want.

This commit also removes the ignore-id-req option since that only makes matter worse than not importing the person.