unmerge digitized videos

[kpsherva]

• examples: https://cds.cern.ch/record/385487, https://cds.cern.ch/record/379116/export/hm?no_redirect_migrated, https://cds.cern.ch/record/410908/export/hm?no_redirect_migrated
• we need to unmerge the records - move videos metadata and file links to a separate record and link them together via field 787 with proper relation description, see https://cds.cern.ch/record/edit/compare_revisions?recid=385487&rev1=20231006150409&rev2=20160622173640 to identify fields
• this step unblocks the migration of research and other documents which have digitisation data mized together, further steps of migrating the new video record must be discussed and executed.