Fix #4901: Duplicate disapprovals

* Add uniqueness constraint on post_disapprovals (user_id, post_id).
* Add fix script to remove existing duplicates.

db/migrate/20211013011619_add_unique_user_id_and_post_id_index_to_post_disapprovals.rb

@@ -0,0 +1,5 @@
+class AddUniqueUserIdAndPostIdIndexToPostDisapprovals < ActiveRecord::Migration[6.1]
+  def change
+    add_index :post_disapprovals, [:user_id, :post_id], unique: true
+  end
+end