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/forum_post.rb

@@ -62,30 +62,7 @@ class ForumPost < ApplicationRecord
     end
   end
 
-  module ApiMethods
-    def as_json(options = {})
-      if CurrentUser.user.level < topic.min_level
-        options[:only] = [:id]
-      end
-
-      super(options)
-    end
-
-    def to_xml(options = {})
-      if CurrentUser.user.level < topic.min_level
-        options[:only] = [:id]
-      end
-
-      super(options)
-    end
-    
-    def hidden_attributes
-      super + [:text_index]
-    end
-  end
-
   extend SearchMethods
-  include ApiMethods
 
   def self.new_reply(params)
     if params[:topic_id]