PERF: Use a separate route for user cards, and split user serializer (#8789)

Adds a new route `/u/{username}/card.json`, which has a reduced number of fields. This change is behind a hidden site setting, so we can test compatibility before rolling out.

config/routes.rb

@@ -478,6 +478,7 @@ Discourse::Application.routes.draw do
     get "#{root_path}/:username/profile-hidden" => "users#profile_hidden"
     put "#{root_path}/:username/feature-topic" => "users#feature_topic", constraints: { username: RouteFormat.username }
     put "#{root_path}/:username/clear-featured-topic" => "users#clear_featured_topic", constraints: { username: RouteFormat.username }
+    get "#{root_path}/:username/card.json" => "users#show_card", format: :json, constraints: { username: RouteFormat.username }
   end
 
   get "user-badges/:username.json" => "user_badges#username", constraints: { username: RouteFormat.username }, defaults: { format: :json }

config/site_settings.yml

@@ -1976,6 +1976,11 @@ uncategorized:
     default: ""
     hidden: true
 
+  enable_new_user_card_route:
+    default: false
+    client: true
+    hidden: true
+
 user_preferences:
   default_email_digest_frequency:
     enum: "DigestEmailSiteSetting"