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_appeal.rb

@@ -9,6 +9,8 @@ class PostAppeal < ApplicationRecord
   before_validation :initialize_creator, :on => :create
   validates_uniqueness_of :creator_id, :scope => :post_id, :message => "have already appealed this post"
 
+  api_attributes including: [:is_resolved]
+
   module SearchMethods
     def resolved
       joins(:post).where("posts.is_deleted = false and posts.is_flagged = false")
@@ -64,8 +66,4 @@ class PostAppeal < ApplicationRecord
   def appeal_count_for_creator
     creator.post_appeals.recent.count
   end
-
-  def method_attributes
-    super + [:is_resolved]
-  end
 end