Make sure all lesson group positions are zero based

[dmcavoy]

Fixes an issue where non-user-facing lesson groups were 0 based when they should be 1 based. Tested locally that the change will update the lesson groups during the seed process.

Testing story

1. Tested seeding locally
2. Added validation that makes sure position is an integer greater than 0
3. Updated the lesson group factory to set position for a new factory created lesson group

Reviewer Checklist:

• Tests provide adequate coverage
• 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

[davidsbailey]

Thank you, @dmcavoy !