DEV: Ship first pass of new user page navigation behind feature flag (#18285)

This commits introduces a new SiteSetting.enable_new_user_profile_nav_groups
feature flag. When configured, users of the configured groups will see
the new user page navigation links.

As of this commit, only the user activity navigation link has been
converted to the newly proposed dropdown of navigation links.

Mobile support has not been considered.

config/site_settings.yml

@@ -2002,6 +2002,14 @@ developer:
     type: tag_list
     default: ""
     client: true
+  enable_new_user_profile_nav_groups:
+    client: true
+    type: group_list
+    list_type: compact
+    default: ""
+    allow_any: false
+    refresh: true
+    hidden: true
 
 embedding:
   embed_by_username: