Encrypt storage secret using scrypt KDF.

* Change format of storage.
* Refactor and modify export/import recovery documents logic.
* Change storage secret length to 512.
* Encrypt the storage document with derived function.
* Refactor property name inside crypto submodule.
* Add docstrings for new methods.
* Fix wrong method call import_recovery_document.
* Fix base64 encoding and secret/kdf length info.
* Add missing header.
* Add missing fields to comment.

Closes 2475 and 2423.
Remove bootstrap param from Soledad.
Add changes file.

* Include info about scrypt and backwards compatibility.
* Add missing 'utf-8' header to test file.