using argon2 instead of bcrypt for password hashing

[zekroTJA]

Argon2id is more secure and up-to-date then bcrypt.

This step requires both hashing algorithms to be implemented. On each new account creation or password change, argon2 will be used instead of bcrypt and bcrypt will only be available for password hashes created before the update.

Using the following package as simple and conveinient wrapper for go's argon2id implementation:
https://github.com/alexedwards/argon2id