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/api/v1/instances/privacy_policies_controller.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class Api::V1::Instances::PrivacyPoliciesController < Api::BaseController
+  skip_before_action :require_authenticated_user!, unless: :whitelist_mode?
+
+  before_action :set_privacy_policy
+
+  def show
+    expires_in 1.day, public: true
+    render json: @privacy_policy, serializer: REST::PrivacyPolicySerializer
+  end
+
+  private
+
+  def set_privacy_policy
+    @privacy_policy = PrivacyPolicy.current
+  end
+end