Fix various encryption details.

* Use AES256 for encrypting storage_secret.
* Add prepended IV to storage secret's ciphertext.
* _get_storage_secret() returns binary.
* Fix key length when hashing using scrypt.
* Remove password from export/import_recovery_document().
Change symmetric encryption scheme to use AES256.
Add check for encryption method.
Increase storage secret size and use half of it to get the local storage passphrase.
Improve _has_secret() logic and tests.
Improve export/import_recovery_document()

* Fix docstrings.