Fix missing/incorrect Model specifications in migrations

We need to define the required models within the migration (not below) to work reliably.

db/migrate/20211114145024_create_tips_intervention.rb

@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class CreateTipsIntervention < ActiveRecord::Migration[6.1]
+  class Intervention < ApplicationRecord
+  end
+
   def change
     Intervention.find_or_create_by(name: 'TipsIntervention')
   end