Fix lessons not being saved when creating a course on the latest Gutenberg version

[m1r0]

Resolves #7083

Proposed Changes

This bug was caused because of the logic detecting if a post is saved does not work on the latest Gutenberg version.

In our logic, we're tracking 3 different save requests - saving the post, saving the post meta, and saving the course structure. The problem was that in the latest Gutenberg version, the post meta saving request starts while the post saving request is still running. That caused a bug because the logic wasn't expecting the requests to happen in parallel. This PR changes the logic to track the requests independently.

I wasn't able to find a good way to add integration tests for this, but it should be covered by E2E tests already.

Testing Instructions

1. Install the latest version of the Gutenberg plugin
2. Go to Sensei LMS -> Courses -> New Course
3. In the Course Outline block, select "Start with a blank canvas"
4. Save the course as draft
5. Make sure the lessons are saved
6. Disable the Gutenberg plugin and make sure the same process works

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[github-actions]

WordPress Dependencies Report

The github-action-wordpress-dependencies-report action has detected some script changes between the commit 83f81de and trunk. Please review and confirm the following are correct before merging.

Script Handle	Added Dependencies	Removed Dependencies	Total Size	Size Diff
blocks/single-course.js			27 kB	-6 B ( -0.03% ⬇️ )
blocks/quiz/index.js			32.2 kB	-3 B ( -0.01% ⬇️ )

This comment was automatically generated by the github-action-wordpress-dependencies-report action.

[codecov]

Codecov Report

Merging #7091 (83f81de) into trunk (26ef3f4) will increase coverage by 0.01%.
Report is 11 commits behind head on trunk.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##              trunk    #7091      +/-   ##
============================================
+ Coverage     49.32%   49.33%   +0.01%     
- Complexity    10538    10539       +1     
============================================
  Files           575      575              
  Lines         44501    44503       +2     
  Branches        402      402              
============================================
+ Hits          21950    21956       +6     
+ Misses        22224    22220       -4     
  Partials        327      327              

Files Changed	Coverage Δ
assets/shared/structure/structure-store.js	80.76% <50.00%> (ø)
...ers/class-sensei-unsupported-theme-handler-cpt.php	85.71% <100.00%> (+7.02%)	⬆️

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[Imran92]

Works as expected 👍