add new model for post replacements, add undo functionality

db/migrate/20170512221200_create_post_replacements.rb

@@ -0,0 +1,14 @@
+class CreatePostReplacements < ActiveRecord::Migration
+  def change
+    create_table :post_replacements do |t|
+    	t.integer :post_id, null: false
+    	t.integer :creator_id, null: false
+    	t.text :original_url, null: false
+    	t.text :replacement_url, null: false
+      t.timestamps null: false
+    end
+
+    add_index :post_replacements, :post_id
+    add_index :post_replacements, :creator_id
+  end
+end