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_topic.rb

@@ -155,26 +155,6 @@ class ForumTopic < ApplicationRecord
     (response_count / Danbooru.config.posts_per_page.to_f).ceil
   end
 
-  def as_json(options = {})
-    if CurrentUser.user.level < min_level
-      options[:only] = [:id]
-    end
-
-    super(options)
-  end
-
-  def to_xml(options = {})
-    if CurrentUser.user.level < min_level
-      options[:only] = [:id]
-    end
-
-    super(options)
-  end
-
-  def hidden_attributes
-    super + [:text_index, :min_level]
-  end
-
   def merge(topic)
     ForumPost.where(:id => self.posts.map(&:id)).update_all(:topic_id => topic.id)
     topic.update(response_count: topic.response_count + self.posts.length, updater_id: CurrentUser.id)