Allow one to "remix" import command

[tarasglek]

I think the cool part of chatcraft is that it's a choose-your-own-adventure chat-dev environment. Currently the way we implement import doesn't quite match the cool dev ux elsewhere.

I find our import command finds sometimes and I have no recourse. I think if we redo it via valtown, we could get a link to the failing val and an option to fork and change it.

"Import command failed, feel free to fork it and fix it via /set import <your_url>" where command could be a link to a val.town func.

I really like how val.town matches our lightweight/interactive code-dev values.