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_flag.rb

@@ -94,22 +94,13 @@ class PostFlag < ApplicationRecord
     end
   end
 
-  module ApiMethods
-    def hidden_attributes
-      list = super
-      unless CurrentUser.can_view_flagger_on_post?(self)
-        list += [:creator_id]
-      end
-      super + list
-    end
-
-    def method_attributes
-      super + [:category]
-    end
+  def api_attributes
+    attributes = super + [:category]
+    attributes -= [:creator_id] unless CurrentUser.can_view_flagger_on_post?(self)
+    attributes
   end
 
   extend SearchMethods
-  include ApiMethods
 
   def category
     case reason