Add translations for lessons and quizzes that were created while saving the course structure

[merkushin]

Resolves #6978

When we create a lesson or a quiz while saving the course structure, the translation is not created in WPML. Unfortunately, I didn't find why it works that way specifically for this case: if you create a course or a lesson in UI, everything work fine. I suppose it might happen with loading meta boxes or saving data from them.

Proposed Changes

• Add translations for lessons and quizzes that were created while saving the course structure

Testing Instructions

1. Install and activate WPML.
2. Go to Sensei LMS > Courses, create a new course.
3. Start with a blank (or default?) course structure (when 3 default lessons pre-filled). That's for simplicity, and you can play at this point.
4. Save draft or publish the course.
5. It is expected that the status of lessons changed from Unsaved to Draft.
6. You can also check if a translation entry was created in database: SELECT * FROM wp_icl_translations WHERE element_id=7686; -- 7686 is Lesson ID

New/Updated Hooks

• sensei_course_structure_lesson_created - Fires after a lesson is created while saving the course structure.
• sensei_course_structure_quiz_created - Fires after a quiz is created while saving the course structure.

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

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (49038f8) 50.94% compared to head (179ff35) 51.21%.
Report is 105 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk    #7383      +/-   ##
============================================
+ Coverage     50.94%   51.21%   +0.26%     
- Complexity    11149    11184      +35     
============================================
  Files           613      614       +1     
  Lines         47062    47214     +152     
  Branches        404      405       +1     
============================================
+ Hits          23975    24179     +204     
+ Misses        22760    22708      -52     
  Partials        327      327              

Files	Coverage Δ
includes/class-sensei-course-structure.php	91.70% <87.50%> (-0.16%)	⬇️
includes/wpml/class-sensei-wpml.php	91.11% <95.00%> (+91.11%)	⬆️

... and 26 files with indirect coverage changes

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[donnapep]

Problem is solved. 👍🏻 Left a few comments. Feel free to merge after addressing.