Fix bootstrap cleanup
As hopefully explained in the comment. The symptom of this was that
bootstrapping would work just fine the first time you called it
in any run of the app, but then if called a second time (eg. if you
cancelled by dismissing the password prompt) it would create keys and
upload the public parts but not store the private parts in SSSS,
leaving you with cross signing keys you don't have the private parts
of.
Also use object.assign in the save keys callback just in case we
ever reset a subset of the keys (and also because it makes it a
bit simpler to reason about what objects are where).
