Scenarios [work in progress]

- [x] Add the concept of Scenarios (Agent Groups)
- [x] Allow Scenarios to be exported as JSON dumps
- [x] Allow Scenarios to be imported
- [x] Repeated import of the same Scenario should offer to update the existing one
- [ ] Show visual diff when the existing Scenario has been edited
Cool, would that also work if we, at some point, had a global scenario repository and let users import scenarios from there?