Move to non extractable keys model, deprecate export-settings

[jchris]

Is your feature request related to a problem? Please describe.
All of our other wallets will use non extractable keys.

Describe the solution you'd like
We should drop the export-settings system in favor of a way to list account and select which one to act as. That way we can eventually move to hardware specific key managers.

It's ok if there is a cleartext key file for the short term, but we should improve that eventually.

Describe alternatives you've considered
Eventually security conscious users will bring their own UCAN from a web wallet or other key management solution.

[jchris]

This is not urgent yet.