chore: Add missing indexes for attachments table (#5588)

- index for attachments table
- index for conversations table

db/migrate/20221010212946_add_index_to_message_attachments.rb

@@ -0,0 +1,8 @@
+class AddIndexToMessageAttachments < ActiveRecord::Migration[6.1]
+  def change
+    add_index :attachments, :account_id
+    add_index :attachments, :message_id
+    add_index :conversations, :contact_id
+    add_index :conversations, :inbox_id
+  end
+end