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/mod_action.rb

@@ -2,6 +2,8 @@ class ModAction < ApplicationRecord
   belongs_to :creator, :class_name => "User"
   before_validation :initialize_creator, :on => :create
 
+  api_attributes including: [:category_id]
+
   #####DIVISIONS#####
   #Groups:     0-999
   #Individual: 1000-1999
@@ -73,10 +75,6 @@ class ModAction < ApplicationRecord
     self.class.categories[category]
   end
 
-  def method_attributes
-    super + [:category_id]
-  end
-
   def serializable_hash(*args)
     super(*args).merge("description" => filtered_description)
   end