DEV: Unhide the set_locale_from_cookie setting to allow site translations (#30924)

Related: https://github.com/discourse/discourse-translator/pull/198

The new experimental feature on discourse-translator requires this
`SiteSetting.set_locale_from_cookie` to be set. This comment unhides the
existing site setting.

config/locales/server.en.yml

@@ -1736,6 +1736,7 @@ en:
     default_locale: "The default language of this Discourse instance. You can replace the text of system generated categories and topics at <a href='%{base_path}/admin/customize/site_texts' target='_blank'>Customize / Text</a>."
     allow_user_locale: "Allow users to choose their own language interface preference"
     set_locale_from_accept_language_header: "set interface language for anonymous users from their web browser's language headers"
+    set_locale_from_cookie: "Allows setting an anonymous user's locale via the 'locale' browser cookie"
     support_mixed_text_direction: "Support mixed left-to-right and right-to-left text directions."
     min_post_length: "Minimum allowed post length in characters (excluding personal messages)"
     min_first_post_length: "Minimum allowed first post (topic body) length (excluding personal messages)"

config/site_settings.yml

@@ -152,7 +152,6 @@ basic:
     validator: "AllowUserLocaleEnabledValidator"
   set_locale_from_cookie:
     default: false
-    hidden: true
     client: true
     validator: "AllowUserLocaleEnabledValidator"
   support_mixed_text_direction: