unique_together integration in CW (alf+syt)
* added in-DB representation of the constraint with a new entity type (CWUniqueTogetherConstraint)
* the schema is now migrated on a sync_schema_props_perms(EType) call in the migration script
* removed the migration commands provided in the first implementation
# type attribute might already be there if migrating from
# version < 3.5 to version >= 3.5.3, BaseTransition being added
# in bootstrap_migration
if versions_map['cubicweb'][0] >= (3, 5, 0):
add_attribute('BaseTransition', 'type')
sync_schema_props_perms('state_of')
sync_schema_props_perms('transition_of')