FEATURE: Automatically grid images when 3 or more image uploads (#29260)

This PR adds the feature where three or more image uploads in the composer will result in the images being surrounded by `[grid]` tags. This helps take advantage of the grid feature (https://github.com/discourse/discourse/pull/21513) and display images in a more appealing way immediately after upload.

config/locales/server.en.yml

@@ -2716,6 +2716,7 @@ en:
     experimental_form_templates: "EXPERIMENTAL: Enable the form templates feature. <b>After enabled,</b> 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: "EXPERIMENTAL: Lazy load category information only for users of these groups. This improves performance on sites with many categories."
+    experimental_auto_grid_images: "EXPERIMENTAL: 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"

config/site_settings.yml

@@ -2471,6 +2471,9 @@ developer:
     default: 50
     hidden: true
     client: true
+  experimental_auto_grid_images:
+    default: false
+    client: true
 
 navigation:
   navigation_menu: