Improved UX for new scenarios

[ggiraldez]

Closes #696

• In the initial scenario, hide the 3-dot options menu with actions for the scenario (most of them do not make sense for the initial scenario)
• Add a legend and a button to create a new scenario in the sidebar
• Hide the "Changes" part of the map legend in the initial scenario
• Improve the naming of newly created scenarios:
  • Copies from the initial scenario are named "Scenario A", "Scenario B" and so on.
  • Use numbers instead if "Scenario Z" already exists, ie. "Scenario 1", "Scenario 2", etc.
  • Copies of other scenarios have a copy number suffix added, ie. "Foo" -> "Foo (1)" and "Foo (3)" -> "Foo (4)"
• Bonus: refactor to reduce duplication in scenario endpoint code