Fix Google Translate breaking web interface (#15610)

- marks the page as a whole as untranslatable
- still marks user text as translatable

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/views/public_timelines/show.html.haml

@@ -14,4 +14,4 @@
     %p= t('about.browse_local_posts')
 
 #mastodon-timeline{ data: { props: Oj.dump(default_props.merge(local: !Setting.show_known_fediverse_at_about_page)) }}
-#modal-container
+.notranslate#modal-container