Change privacy policy to be rendered in web UI, add REST API (#19310)

Source string no longer localized, Markdown instead of raw HTML

app/controllers/privacy_controller.rb

@@ -1,22 +1,11 @@
 # frozen_string_literal: true
 
 class PrivacyController < ApplicationController
-  layout 'public'
-
-  before_action :set_instance_presenter
-  before_action :set_expires_in
+  include WebAppControllerConcern
 
   skip_before_action :require_functional!
 
-  def show; end
-
-  private
-
-  def set_instance_presenter
-    @instance_presenter = InstancePresenter.new
-  end
-
-  def set_expires_in
-    expires_in 0, public: true
+  def show
+    expires_in 0, public: true if current_account.nil?
   end
 end