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/comment.rb

@@ -22,6 +22,8 @@ class Comment < ApplicationRecord
     :body => ->(user_name) {"@#{creator.name} mentioned you in a \"comment\":/posts/#{post_id}#comment-#{id} on post ##{post_id}:\n\n[quote]\n#{DText.excerpt(body, "@"+user_name)}\n[/quote]\n"},
   )
 
+  api_attributes including: [:creator_name, :updater_name]
+
   module SearchMethods
     def deleted
       where("comments.is_deleted = true")
@@ -145,14 +147,6 @@ class Comment < ApplicationRecord
     select { |comment| comment.visibility(user) == :visible }
   end
 
-  def hidden_attributes
-    super + [:body_index]
-  end
-
-  def method_attributes
-    super + [:creator_name, :updater_name]
-  end
-
   def creator_name
     creator.name
   end