api: refactor api attribute declarations.

Replace the `method_attributes` and `hidden_attributes` methods with
`api_attributes`. `api_attributes` can be used as a class macro:

    # include only the given attributes.
    api_attributes :id, :created_at, :creator_name, ...

    # include all default attributes plus the `creator_name` method.
    api_attributes including: [:creator_name]

or as an instance method:

    def api_attributes
       [:id, :created_at, :creator_name, ...]
    end

By default, all attributes are included except for IP addresses and
tsvector columns.

app/models/pool.rb

@@ -17,6 +17,8 @@ class Pool < ApplicationRecord
   after_save :create_version
   after_create :synchronize!
 
+  api_attributes including: [:creator_name, :post_count]
+
   module SearchMethods
     def deleted
       where("pools.is_deleted = true")
@@ -288,10 +290,6 @@ class Pool < ApplicationRecord
     (post_count / CurrentUser.user.per_page.to_f).ceil
   end
 
-  def method_attributes
-    super + [:creator_name, :post_count]
-  end
-
   def creator_name
     creator.name
   end