user creation api extension: Added temp password feature from admin ui into users api call

This pull request
* fixes outdated comments in the POST /users api call
* adds the parameter expired_password to the call which if set will cause the password to be set expired
* adds the parameter force_random_password to the call which if set will cause the password to be randomly generated

To properly handle the more complex logic needed to make this change backwards-compatible and robust, some code had to be added. Comments were inserted where expedient.

I believe it is self-evident that adding the "generate temp password" feature that is already in the admin ui to the api call is useful. It enables for example batch-adding users with random passwords.