Implement mnemonic export and import

[RiccardoM]

Context

Currently when logging in the user is only able to perform the mnemonic login by manually typing each and every word of the mnemonic itself. At the same time, once he has logged in, he can only see his mnemonic and manually write it down on a piece of paper.

Feature description

In order to facilitate the export and later login using the mnemonic, what we could do is allowing him to export it as a shielded file. The overall working could then be:

1. The user logs in using an existing mnemonic or creates a new account.
2. Once logged in, he exports his mnemonic by protecting it with a password.
3. When he wants to log in again on another device, he can use a new option that requires him to input the file and the password to unlock it.

[RiccardoM]

Closed with 76e0606