Add option to opt out of search engines on public profile/status pages (#4199)

config/locales/simple_form.en.yml

@@ -12,6 +12,7 @@ en:
         note:
           one: <span class="note-counter">1</span> character left
           other: <span class="note-counter">%{count}</span> characters left
+        setting_noindex: Affects your public profile and status pages
       imports:
         data: CSV file exported from another Mastodon instance
       sessions:
@@ -41,6 +42,7 @@ en:
         setting_default_sensitive: Always mark media as sensitive
         setting_delete_modal: Show confirmation dialog before deleting a toot
         setting_system_font_ui: Use system's default font
+        setting_noindex: Opt-out of search engine indexing
         severity: Severity
         type: Import type
         username: Username

config/settings.yml

@@ -21,6 +21,7 @@ defaults: &defaults
   auto_play_gif: false
   delete_modal: true
   system_font_ui: false
+  noindex: false
   notification_emails:
     follow: false
     reblog: false