Migrates from ranked-model to positioning gem

[rosschapman]

@anaulin and I spent an ensemble pairing on this but eventually ran into issues when manually testing creation of new Gizmos.

• ✅ The migrations and installation of the positioning code is sound
• 😢 But, we got a NoMethodError that position_will_change! does not exist on the furniture instance when execution reaches furniture.save in the controller
• 🤔 We were curious about it being an issue with STI models, but the gem includes a test case for STI
• 🤔 We were also curious about whether our custom Furniture model code might be interacting with the gem in a strange way
• ⏭️ Thought: rather than hack our way around the Furniture model, we can put this code on standby while we refactor the furniture STI models into standalone models.