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
Neat! Did not know `first_or_initialize` existed, much nicer syntax then using `find_or_initialize_by`