Make default user level and settings configurable.

Remove start_as_gold? and start_as_contributor? from default config
because they don't actually do anything. `git log -G start_as` indicates
they never have done anything.

Add a more general customize_new_user method for setting the starting
user level, permissions and defaults for any other settings.

app/models/user.rb

@@ -64,6 +64,7 @@ class User < ActiveRecord::Base
   after_save :update_cache
   after_update :update_remote_cache
   before_create :promote_to_admin_if_first_user
+  before_create :customize_new_user
   #after_create :notify_sock_puppets
   has_many :feedback, :class_name => "UserFeedback", :dependent => :destroy
   has_many :posts, :foreign_key => "uploader_id"
@@ -311,6 +312,10 @@ class User < ActiveRecord::Base
       end
     end
 
+    def customize_new_user
+      Danbooru.config.customize_new_user(self)
+    end
+
     def role
       case level
       when Levels::MEMBER