Add "ALLOW_DEMO_RESET" env variable, default FALSE

[taylordowns2000]

As far as "foot guns" go, this is a big one. I'd prefer it if most lightning instances could not be reset to a demo state with a single command.

• Use the Vapor API
• Make sure that if nothing gets set, this thing is false. (It's a special case when someone would actually want a "destroy everything" button... we happen to use it for our public sandbox which gets a nightly cleaning.)
• Expect Lightning.Demo.reset_demo() throws an exception if someone attempts it without this ENV flag
• Expect the mix task to throw an exception also