[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.

[server] commit after serializing schema Use separate transactions for schema serialization and postcreate scripts execution, so that the latter can rely on the schema being persistently stored. This matters e.g. if something in a postcreate violates a unique_together constraint, we need the constraint persisted in order to raise a meaningful error. See #4525069.

[book] fix docstrings to please sphinx example rst roles in docstring must be protected (in a literal).