cubicweb-conference #1177703 add information that registration is finished [open]

For a paying confernce, when we go to a talk (and that registration is finished), we still have the button to plan to attend to the talk (which does nothing). We must have a message instead to inform that registration is finished

priorityminor
typebug
done in0.15.0
load0.250
load left0.250
closed by<not specified>