Change join table to own list_items table
Create ListItem model and update associations
Drop, recreate and reseed db
Change routes to my/list/list_items
Rename my/items_controller.rb to my/list_items_controller.rb
Adjust my/list_items_controller logic for checkbox input
Adjust my/lists/show.html.erb with new association logic
Refactor jQuery 'details expand' logic to add 'selected' class