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

Source string no longer localized, Markdown instead of raw HTML

app/javascript/mastodon/features/ui/util/async-components.js

@@ -169,3 +169,7 @@ export function FilterModal () {
 export function About () {
   return import(/*webpackChunkName: "features/about" */'../../about');
 }
+
+export function PrivacyPolicy () {
+  return import(/*webpackChunkName: "features/privacy_policy" */'../../privacy_policy');
+}