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

Source string no longer localized, Markdown instead of raw HTML

app/serializers/rest/privacy_policy_serializer.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class REST::PrivacyPolicySerializer < ActiveModel::Serializer
+  attributes :updated_at, :content
+
+  def updated_at
+    object.updated_at.iso8601
+  end
+
+  def content
+    markdown.render(object.text % { domain: Rails.configuration.x.local_domain })
+  end
+
+  private
+
+  def markdown
+    @markdown ||= Redcarpet::Markdown.new(Redcarpet::Render::HTML, escape_html: true, no_images: true)
+  end
+end