fix: Notification page breakages (#5236)

- Remove the cascading foreign key indexes
- Add migration to clean up existing objects

fixes: #4285

app/models/category.rb

@@ -23,11 +23,6 @@
 #  index_categories_on_parent_category_id             (parent_category_id)
 #  index_categories_on_slug_and_locale_and_portal_id  (slug,locale,portal_id) UNIQUE
 #
-# Foreign Keys
-#
-#  fk_rails_...  (associated_category_id => categories.id)
-#  fk_rails_...  (parent_category_id => categories.id)
-#
 class Category < ApplicationRecord
   belongs_to :account
   belongs_to :portal