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/post_archive.rb

@@ -260,7 +260,7 @@ class PostArchive < ApplicationRecord
     updater.name
   end
 
-  def method_attributes
+  def api_attributes
     super + [:obsolete_added_tags, :obsolete_removed_tags, :unchanged_tags, :updater_name]
   end