[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()