Replace highline dependency with standard Ruby

There are two minor differences from the `highline` version.

* Character echoing on password input has been disabled altogether.

  This is due to limitations of the `console/io` library.

* The password length validation has been removed.

  There's no feature like this in the `console/io` library and there's no validation like this on the model. Instead of reimplementing it, I think we might as well remove it.

The test for `ChangePassword` could use some improvement, especially to avoid stubbing a method on the object under test.

The change to `ChangeUserPassword` is tangential, but fixes a bug where the password wouldn't be updated due to a failing validation (`password` and `password_confirmation` don't match).

Ref: #334