DEV: Include unread topics in New topic lists and link to it in sidebar (#20432)

This commit introduces a few experimental changes to the New topics list and "Everything" link in the sidebar:

1. Make the New topics list include unread topics
2. Make the Everything section in the sidebar link to the New topics list (`/new`)
3. Remove "unread" or "new" text next to the count and keep the count
4. The count is a sum of new and unread topics counts

All of these of changes are behind an off-by-default feature flag. I've not written extensive tests for these changes because they're highly experimental.

Internal topic: t/77234.

config/locales/server.en.yml

@@ -2398,6 +2398,7 @@ en:
     default_sidebar_tags: "Selected tags will be displayed under Sidebar's Tags section by default."
     enable_new_notifications_menu: "Enables the new notifications menu for the legacy navigation menu."
     enable_experimental_hashtag_autocomplete: "EXPERIMENTAL: Use the new #hashtag autocompletion system for categories and tags that renders the selected item differently and has improved search"
+    experimental_new_new_view_groups: "EXPERIMENTAL: Enable a new topics list that combines unread and new topics and make the \"Everything\" link in the sidebar link to it."
 
     errors:
       invalid_css_color: "Invalid color. Enter a color name or hex value."

config/site_settings.yml

@@ -2073,6 +2073,13 @@ developer:
     client: true
     default: false
     hidden: true
+  experimental_new_new_view_groups:
+    client: true
+    type: group_list
+    list_type: compact
+    default: ""
+    allow_any: false
+    refresh: true
 
 navigation:
   navigation_menu: