[migration] hackish black magic to bootstrap addition of formula attr during migration Turns out we can't add an attribute to CWAttribute before the attribute exists. add_attribute generates rql with a 'formula' relation, which CW doesn't know about yet, so things get unhappy. Once that is fixed, we need to make the CWAttribute addition operation deal with CWAttribute entities that don't have a formula yet. Finally, move the migration to bootstrapmigration_repository so it happens early on, and use add_entity_type rather than add_relation_type for CWComputedRType.