Swapping the lessons list for a course to use the course ID so it can be used on the single lesson page also

[kimcoleman]

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Changes proposed in this Pull Request:

The function that shows the lesson list for a single course (including whether its completed or not) works great on the single course page (in the context of that global $post / $post->ID.

I wanted to use this same function to output the lesson list on a lesson within the course - but the status of whether it was complete wasn't showing.

This change should not affect any functionality and only opens it up to checking the context of the passed $course_id which is already a requirement of rendering the output anyway.

How to test the changes in this Pull Request:

1. Add the function to output the lessons html to a single lesson template file: echo pmpro_courses_get_lessons_html( $post->post_parent );
2. without the change, the status of whether lessons are complete is not available.
3. with the change, the correct lesson status is shown in the list.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

• BUG FIX/ENHANCEMENT: Now using $course_id in outputting lesson list in the context of a specific course.

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.