[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.