Test product model - price change threshold method returns true when the price change is between 5% - 30% and false otherwise
Test product model - price_stable? method checks to see if the price has changed within the past 30 days. If it has return false if not return true.
change product table - change column red_pencil_tag to  red_pencil_tag_started_at
Add method .tag_sale_over? to product model
Add method .price_increased? to product model
Add RedPencilTag model and refactor Product model methods to work with the new model
Refactor product tests
Start sale if price change is within threshold on save