FIX: category selectors for lazy loaded categories (#24533)

A lot of work has been put in the select kits used for selecting
categories: CategorySelector, CategoryChooser, CategoryDrop, however
they still do not work as expected when these selectors already have
values set, because the category were still looked up in the list of
categories stored on the client-side Categrories.list().

This PR fixes that by looking up the categories when the selector is
initialized. This required altering the /categories/find.json endpoint
to accept a list of IDs that need to be looked up. The API is called
using Category.asyncFindByIds on the client-side.

CategorySelector was also updated to receive a list of category IDs as
attribute, instead of the list of categories, because the list of
categories may have not been loaded.

During this development, I noticed that SiteCategorySerializer did not
serializer all fields (such as permission and notification_level)
which are not a property of category, but a property of the relationship
between users and categories. To make this more efficient, the
preload_user_fields! method was implemented that can be used to
preload these attributes for a user and a list of categories.

app/models/category.rb

@@ -109,7 +109,6 @@ class Category < ActiveRecord::Base
 
   after_save :reset_topic_ids_cache
   after_save :clear_subcategory_ids
-  after_save :clear_parent_ids
   after_save :clear_url_cache
   after_save :update_reviewables
   after_save :publish_discourse_stylesheet
@@ -130,7 +129,6 @@ class Category < ActiveRecord::Base
 
   after_destroy :reset_topic_ids_cache
   after_destroy :clear_subcategory_ids
-  after_destroy :clear_parent_ids
   after_destroy :publish_category_deletion
   after_destroy :remove_site_settings
 
@@ -227,6 +225,34 @@ class Category < ActiveRecord::Base
   # Allows us to skip creating the category definition topic in tests.
   attr_accessor :skip_category_definition
 
+  def self.preload_user_fields!(guardian, categories)
+    category_ids = categories.map(&:id)
+
+    # Load notification levels
+    notification_levels = CategoryUser.notification_levels_for(guardian.user)
+    notification_levels.default = CategoryUser.default_notification_level
+
+    # Load permissions
+    allowed_topic_create_ids =
+      if !guardian.is_admin? && !guardian.is_anonymous?
+        Category.topic_create_allowed(guardian).where(id: category_ids).pluck(:id).to_set
+      end
+
+    # Categories with children
+    with_children =
+      Category.where(parent_category_id: category_ids).pluck(:parent_category_id).to_set
+
+    # Update category attributes
+    categories.each do |category|
+      category.notification_level = notification_levels[category[:id]]
+
+      category.permission = CategoryGroup.permission_types[:full] if guardian.is_admin? ||
+        allowed_topic_create_ids&.include?(category[:id])
+
+      category.has_children = with_children.include?(category[:id])
+    end
+  end
+
   def self.topic_id_cache
     @topic_id_cache ||= DistributedCache.new("category_topic_ids")
   end
@@ -859,24 +885,9 @@ class Category < ActiveRecord::Base
     self.where("string_to_array(email_in, '|') @> ARRAY[?]", Email.downcase(email)).first
   end
 
-  @@has_children = DistributedCache.new("has_children")
-
-  def self.has_children?(category_id)
-    @@has_children.defer_get_set(category_id.to_s) do
-      Category.where(parent_category_id: category_id).exists?
-    end
-  end
-
   def has_children?
-    !!id && Category.has_children?(id)
-  end
-
-  def self.clear_parent_ids
-    @@has_children.clear
-  end
-
-  def clear_parent_ids
-    Category.clear_parent_ids
+    @has_children ||= (id && Category.where(parent_category_id: id).exists?) ? :true : :false
+    @has_children == :true
   end
 
   def uncategorized?