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.