Add subscription model and link it to RFC

db/migrate/20170906124500_create_subscriptions.rb

@@ -0,0 +1,11 @@
+class CreateSubscriptions < ActiveRecord::Migration
+  def change
+    create_table :subscriptions do |t|
+      t.belongs_to :user, polymorphic: true
+      t.references :request_for_comment
+      t.string :type
+
+      t.timestamps null: false
+    end
+  end
+end