Default to password hashing

[Alkarex]

Suggestion by @quackzar:

Currently the plugin does not provide any methods of hashing stored passwords themselves, but delegates
that to the user. However that risks that the user might provide the easier route of using plaintext
passwords, which is discouraged. Therefore a recommend change is to hash inputted passwords by default,
possibly with an opt-out mechanic or partly disallowing plaintext password storage. In this regard it
is important perform salting, however modern password hashing algorithms, such as bcrypt or argon2id
salt passwords automatically.