cubicweb-conference #1251202 add a transition accepted -> rejected for a talk [open]

When a talk is in state "accepted", it is no more possible to reject it. When a talk is cancelled, we must be able to change the talk's state in another state than "accepted", "rejected" for example

prioritynormal
typeenhancement
done in0.15.0
load0.250
load left0.250
closed by<not specified>