DEV: Move image auto-grid back to experimental (#31296)

This feature was originally experimental. We made it baseline in #29572, however this was met with some pushback, so we have decided to make it experimental again.

The original PR couldn't be reverted, but this one basically does the same by cherry-picking all the changes back.

config/locales/server.en.yml

@@ -2755,7 +2755,7 @@ en:
     experimental_form_templates: "Enable the form templates feature. Manage the templates at <a href='%{base_path}/admin/customize/form-templates'>Customize / Templates</a>."
     admin_sidebar_enabled_groups: "Enable sidebar navigation for the admin UI for the specified groups, which replaces the top-level admin navigation buttons."
     lazy_load_categories_groups: "Lazy load category information only for users of these groups. This improves performance on sites with many categories."
-
+    experimental_auto_grid_images: "Automatically wraps images in [grid] tags when 3 or more images are uploaded in the composer."
     page_loading_indicator: "Configure the loading indicator which appears during page navigations within Discourse. 'Spinner' is a full page indicator. 'Slider' shows a narrow bar at the top of the screen."
     show_user_menu_avatars: "Show user avatars in the user menu"
     about_page_hidden_groups: "Do not show members of specific groups on the /about page."

config/site_settings.yml

@@ -3514,6 +3514,9 @@ dashboard:
     client: true
 
 experimental:
+  experimental_auto_grid_images:
+    default: false
+    client: true
   experimental_rename_faq_to_guidelines:
     default: false
     hidden: true