fix: Notification page breakages (#5236)

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

fixes: #4285

app/models/campaign.rb

@@ -28,11 +28,6 @@
 #  index_campaigns_on_inbox_id         (inbox_id)
 #  index_campaigns_on_scheduled_at     (scheduled_at)
 #
-# Foreign Keys
-#
-#  fk_rails_...  (account_id => accounts.id) ON DELETE => cascade
-#  fk_rails_...  (inbox_id => inboxes.id) ON DELETE => cascade
-#
 class Campaign < ApplicationRecord
   include UrlHelper
   validates :account_id, presence: true