Fix reordering of lessons via drag-and-drop on the script edit gui page

[davidsbailey]

Fixes the issue where reordering lessons doesn't work. the root cause is that the lesson ordering code expects position to be relative to the lesson group (parent), but the lesson position is currently set relative to the unit (grandparent).

Specific changes:

• set lesson position relative to lesson group, not unit
• remove unused relativePosition field
• remove unused moveLesson redux action
• eliminate unnecessary references to lesson position, now that the first lesson in each lesson group has position 1

Testing story

• added test cases for reorderLesson redux action.
• manually verified that various dragging scenarios work when multiple lesson groups are present.

Reviewer 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

[dmcavoy]

Did you check that script saving works?

[davidsbailey]

Did you check that script saving works?

Yep! I think it should be possible to add a UI test for this soon.

[dmcavoy]

Looks good!