REFACTOR: Serve auth provider information in the site serializer.

At the moment core providers are hard-coded in Javascript, and plugin providers get added to the JS payload at compile time. This refactor means that we only ship enabled providers to the client.

lib/auth/auth_provider.rb

@@ -0,0 +1,28 @@
+class Auth::AuthProvider
+  include ActiveModel::Serialization
+
+  def initialize(params = {})
+    params.each { |key, value| send "#{key}=", value }
+  end
+
+  def self.auth_attributes
+    [:pretty_name, :title, :message, :frame_width, :frame_height, :authenticator,
+     :pretty_name_setting, :title_setting, :enabled_setting, :full_screen_login, :full_screen_login_setting,
+     :custom_url]
+  end
+
+  attr_accessor(*auth_attributes)
+
+  def name
+    authenticator.name
+  end
+
+  def can_connect
+    authenticator.can_connect_existing_user?
+  end
+
+  def can_revoke
+    authenticator.can_revoke?
+  end
+
+end