fix: Notification page breakages (#5236)

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

fixes: #4285

app/models/message.rb

@@ -85,8 +85,9 @@ class Message < ApplicationRecord
   belongs_to :contact, required: false
   belongs_to :sender, polymorphic: true, required: false
 
-  has_many :attachments, dependent: :destroy_async, autosave: true, before_add: :validate_attachments_limit
+  has_many :attachments, dependent: :destroy, autosave: true, before_add: :validate_attachments_limit
   has_one :csat_survey_response, dependent: :destroy_async
+  has_many :notifications, as: :primary_actor, dependent: :destroy_async
 
   after_create_commit :execute_after_create_commit_callbacks