Move the lessons to a temporary lesson group so that moving lesson groups works

[dmcavoy]

We got a report that moving the last lesson group up was giving errors about legacy script levels not being allowed in a bunch of lessons. The issue is a follow up issue to the fix in this PR: #39255.

Because the fix in the previous PR moved all the lessons to the last lesson group when you moved the last lesson group up it deleted all the lessons that had not been set up in previous lesson groups and that resulted in the chain of issues listed in that previous PR.

To fix this we add a temporary lesson group to hold all the lessons and put it at the end of the script. This temp lesson group will get deleted by these lines in add_script once everything else in the script has been set up.

code-dot-org/dashboard/app/models/script.rb

Lines 1057 to 1060 in 3007647

	temp_lgs = LessonGroup.add_lesson_groups(raw_lesson_groups, script, new_suffix, editor_experiment)
	script.reload
	script.lesson_groups = temp_lgs
	script.save!

Testing story

Added a test to make sure you could move the last lesson group in a migrated script up without lessons getting deleted.

Follow-up work

We still want to move off of this system going forward but after our April deadline.

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked