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/user_name_change_request.rb

@@ -67,11 +67,9 @@ class UserNameChangeRequest < ApplicationRecord
     end
   end
 
-  def hidden_attributes
-    if CurrentUser.is_admin? || user == CurrentUser.user
-      []
-    else
-      super + [:change_reason, :rejection_reason]
-    end
+  def api_attributes
+    attributes = super
+    attributes -= [:change_reason, :rejection_reason] unless CurrentUser.is_admin? || user == CurrentUser.user
+    attributes
   end
 end