Editor saves too many times when saving lessons from the Course Outline block

[donnapep]

Steps to Reproduce

1. Create a new course.
2. In the Course Outline block, click Save to edit lesson from the toolbar.

What I Expected

The editor to save once.

What Happened Instead

The editor appears to be saving multiple times, probably once per lesson.

Screenshot / Video

https://github.com/Automattic/sensei/assets/1190420/25834d0d-9d7d-4f33-a4f0-ced58d872443

[renatho]

In a quick test, I tried to add many lessons to a course, and it didn't increase the number of savings.

IIRC, it should save 2 times only:

• The click on the button that would trigger the original save.
• After that, the course outline should call an endpoint to create the lessons, pull their IDs, and update some block attributes. In this case, it would need to fire another save (hasUnsavedServerUpdates would be true) to save the new attributes.

So I'm not sure why we have 3 saves. The reference where it's probably happening is here: assets/shared/structure/structure-store.js.

If it's helpful, at some point I wrote assets/shared/helpers/editor-lifecycle.js, which could be useful in a refactor to simplify the subscribeToPostSave.