Updating user's PasswordHash to be base64 URL decoded before user import

[thomasmburke]

Developers are having trouble migrating users because of the issue described here. This update looks to update the snippet used in https://firebase.google.com/docs/auth/admin/import-users#import_users_with_firebase_scrypt_hashed_passwords to help users migrate users and avoid issues surrounding double base64 encoding.

[thomasmburke]

So the update for this doc snippet updating two things:
1.) Decoding the user password hash
- currently we don't so when a dev performs import the password hash is getting double encoding leading to invalid password errors when the correct password is provided for a newly imported user
2.) Updating the decoding functions so the hash.Scrypt key uses standard base64 encoding and user password hashes uses base64URL encoding
- currently the snippet doesn't decode the use password hash, but it should using a base64URL encoding

[lahirumaramba]

Thank you @thomasmburke ! LGTM