Allow to create draft steps

[ceolinwill]

Scenario: We have published a course.

Action: We want to add extra steps.

Problem: The new step might break a lesson if someone is playing it.

We should have a way to have draft steps that only show up on a lesson after we publish them. Right now, they’re visible the moment we create them.

Note: This should be applicable only for published lessons. For unpublished lessons, steps should be published by default.