Main reason: https://bugzilla.mozilla.org/show_bug.cgi?id=1133698
Unfortunately, because of this bug Firefox cannot save non-exportable
keys to IndexedDB. As a workaround, we call exportKey before saving and
store JsonWebKey instead. We use importKey again while reading the key
from the database.
Extra reason for this change - during testing isomorphic-webcrypto
complains about keys read from IndexedDB.
While being less secure, this could also help users export their keys if
they want to migrate their identities to another computer...
