Add WKD policy endpoint

spec/requests/web_key_directory_spec.rb

@@ -1,6 +1,14 @@
 require 'rails_helper'
 
 RSpec.describe "OpenPGP Web Key Directory", type: :request do
+  describe "policy" do
+    it "returns an empty 200 response" do
+      get "/.well-known/openpgpkey/policy"
+      expect(response).to have_http_status(:ok)
+      expect(response.body).to be_empty
+    end
+  end
+
   describe "non-existent user" do
     it "returns a 404 status" do
       get "/.well-known/openpgpkey/hu/fmb8gw3n4zdj4xpwaziki4mwcxr1368i?l=aristotle"