id_ext needs upgrade

[carl3]

The id_ext attribute allows correlation of IDs between different external systems. The current value is a single ID (used in the SF-specific omniballot/wineds conversions). However, there are multiple IDs that need to be matched and recorded, plus a future extension should support OCDID and ANSI/FIPS codes.

Rather than a single ID, this should be a list of IDs with a prefix indicating the origin. A separate table can provide info for each prefix used.

External IDs allow data conversion/manipulation software to keep track of the same object across infosystems.