Skip on existing Splocalecontaineritem instead of Splocalecontainer

[acwhite211]

Fixes #7559

Skip on existing Splocalecontaineritem instead of Splocalecontainer. This works on the example workshop database. More testing to be done on other databases.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list

Testing instructions

• Add a new discipline to the database.
• Make sure that the ve/bin/python manage.py run_key_migration_functions Django command runs before testing.
• See that no new schema config related errors occur in the logs for Django command.
• See that new schema config records were created for the new dicpline in the schema config viewer. Make sure to check the newer tables, like collectionobjectgroup and collectionobjecttype.