[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

if repo.system_source.dbdriver == 'postgres':
    sql('ALTER TABLE appears ADD COLUMN weight float')
    sql('UPDATE appears SET weight=1.0 ')