Add Web Key Directory endpoint

Serve public keys in binary and armored text, if they contain a user's
account address.

app/controllers/web_key_directory_controller.rb

@@ -0,0 +1,34 @@
+class WebKeyDirectoryController < WellKnownController
+  before_action :allow_cross_origin_requests, only: [ :show ]
+
+  # /.well-known/openpgpkey/hu/:hashed_username(.txt)
+  def show
+    @user = User.find_by(cn: params[:l])
+
+    if @user.nil? ||
+       @user.pgp_pubkey.empty? ||
+       !@user.pgp_pubkey_contains_user_address?
+      http_status :not_found and return
+    end
+
+    if params[:hashed_username] != @user.wkd_hash
+      http_status :unprocessable_entity and return
+    end
+
+    respond_to do |format|
+      format.text do
+        response.headers['Content-Type'] = 'text/plain'
+        render plain: @user.pgp_pubkey
+      end
+
+      format.any do
+        key = @user.gnupg_key.export
+        send_data key, filename: "#{@user.wkd_hash}.pem",
+                       type: "application/octet-stream"
+      end
+    end
+  end
+
+  private
+
+end