deprecate Transition.set_transition_permissions in favor of set_permissions (after all we know we're on a transition entity)

introduce 'go back' transition: transition without destination state will go to the state we were coming from

fix optimisation with super session that may lead to integrity loss at some point I've decided to stop ensuring ?1 cardinality was respected when adding a new relation using a super session, to avoid the cost of the delete query. That was yet discussable because it introduced unexpected difference between execute and unsafe_execute, which is imo not worth it. Also, now that rql() in migration script default to unsafe_execute, we definitly don't want that implicit behaviour change (which already cause bug when for instance adding another default workflow for an entity type: without that fix we end up with *two* default workflows while the schema tells we can have only one. IMO we should go to the direction that super session skip all security check, but nothing else, unless explicitly asked.