New dissatisfaction and retained customers metrics

Two new endpoints join the Metrics API:

GET /stats/dissatisfaction: number of dissatisfaction alerts and how they were handled (pending, call, text, ignored, spam), with counts and percentages;
GET /stats/retained_customers: number and percentage of retained customers, i.e. dissatisfied customers who, after your reply, said they were satisfied with how their issue was handled and would come back.