Replace OEmbed and initial state Rabl templates with serializers (#4110)

* Replace OEmbed Rabl template with serializer

* Replace initial state rabl with serializer

app/presenters/initial_state_presenter.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class InitialStatePresenter < ActiveModelSerializers::Model
+  attributes :settings, :token, :current_account, :admin
+end