Update SORN field to reference #, not Federal Register URLs

[rebeccawilliams]

The URL may become out of date and need to be update and the # would be helpful for cross referencing data.

cc @justgrimes

[dsmorgan77]

See #223

[justgrimes]

@dsmorgan77 your original pull request didn't seem to get addressed in the previous schema update. The discussion #223 was around including agency identifier for system of record in addition to FRN URL. Given the discussion it seems that FRN URLs is a sub par way to represent System of Record information in the current schema. It sounds like the field should just contain the unique agency identifier for that system of record instead of FRN URL or the proposed FRN URL + agency identifier.