Update dates that archival content was added to CaltechDATA

Dates that archival content were added to CaltechDATA was listed in the "Updated" date field in the old version of CaltechDATA. This didn't really make sense, and didn't get transferred to the new version of CaltechDATA. Figure out what date type makes sense, transfer from backup, and update https://github.com/caltechlibrary/caltechdata_usage