Manage core users

[clue]

It would be nice if we could expose (or at the very least describe) a way to manage user accounts (i.e. add new users and change passwords).

The quassel-core CLI exposes the following options:

$ docker run -it --rm clue/quassel-core --help
Usage: quasselcore [arguments]
     --add-user                             Starts an interactive session to add a new core user
     --change-userpass=[CHANGE-USERPASS]    Starts an interactive session to change the password of the user identified by username
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