Merge: Opportunity: intro ∇ ratio

Add the ∇ ratio to Opportunity with a customizable multiplier. It assigns weight to the choices of participants so that someone who select all possible answers puts less weight on each answer, and someone who choses a single answer puts her full weight on it.

It can be used to count the number of pizzas to order (with a multiplier of 0.4) and to give more importances to the available times of busy people when scheduling a meeting.

Pull-Request: #1783
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>