Add argon2id support and make it default for password hashing

[jappeace]

This appears to be the current recommendation for password hashing:

• https://security.stackexchange.com/questions/193351/in-2018-what-is-the-recommended-hash-to-store-passwords-bcrypt-scrypt-argon2/197550#197550
• https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html

We should still keep scrypt support for legacy systems who can't migrate.