Store hashed passwords (#7)

Use if let instead of unwrap

Default to invalid password if could not calculate

Move hash password methdo and return Result

Rename get_password method

Default to empty password when no pwd is received

Store hashed passwords

Store passwords hashed with Argon2 and verify password with that stored
hash.

Co-authored-by: Guillem Nieto <gnieto.talo@gmail.com>