safe destroy

[kr]

Make hk destroy work only if the app has no:

• addons
• dynos (i.e. it must be scaled to 0 for all ps types)

It's okay to do this in the client as a separate check. There's a race, but I'm not super worried about that.

Ideally this would also make it easier to support undo, in case the user needs to recover their code. But that would need help from Heroku.