Optional Supervisor Notification when user volunteers

Uses ui.config to switch on/off, defaults to off:
    "supervisorEmail": {
      "useSupervisorEmail": false
    }
Saves provided supervisor name/email in usersetting table
findVolunteerSupervisor returns empty when UIConfig.supervisorEmail.useSupervisorEmail is false, uses a user id of 0 when the feature is on.

Addresses #274 
definitely via `require`