FEATURE: New route for loading multiple user cards simultaneously (#9078)

Introduces `/user-cards.json`

Also allows the client-side user model to be passed an existing promise when loading, so that multiple models can share the same AJAX request

app/controllers/users_controller.rb

@@ -99,6 +99,30 @@ class UsersController < ApplicationController
     show(for_card: true)
   end
 
+  def cards
+    return redirect_to path('/login') if SiteSetting.hide_user_profiles_from_public && !current_user
+
+    user_ids = params.require(:user_ids).split(",").map(&:to_i)
+    raise Discourse::InvalidParameters.new(:user_ids) if user_ids.length > 50
+
+    users = User.where(id: user_ids).includes(:user_option,
+                                              :user_stat,
+                                              :default_featured_user_badges,
+                                              :user_profile,
+                                              :card_background_upload,
+                                              :primary_group,
+                                              :primary_email
+                                            )
+
+    users = users.filter { |u| guardian.can_see_profile?(u) }
+
+    preload_fields = User.whitelisted_user_custom_fields(guardian) + UserField.all.pluck(:id).map { |fid| "#{User::USER_FIELD_PREFIX}#{fid}" }
+    User.preload_custom_fields(users, preload_fields)
+    User.preload_recent_time_read(users)
+
+    render json: users, each_serializer: UserCardSerializer
+  end
+
   def badges
     raise Discourse::NotFound unless SiteSetting.enable_badges?
     show