[migration] reorder system tables / meta-schema migration into bootstrap_migration Also fix a bug in the backwards-compat eid_type_source method where it returned an extid (possibly null) instead of a source name. With these changes, migrating a database from cw 3.12 is now possible. Closes #4962681

for table, column in [
        ('transactions', 'tx_time'),
        ('tx_entity_actions', 'tx_uuid'),
        ('tx_relation_actions', 'tx_uuid')]:
    repo.system_source.create_index(session, table, column)

commit()