Improve RSS entries for statuses (#13592)

* Improve RSS entries for statuses

- Render polls in both accounts and tags serializers
- Refactor RSS serializers
- Change title preview to include ellipsis when truncated
- Change title preview to show CW instead of toot text
- Add tests

* Remove title from OEmbed serialization

Twitter doesn't serialize title either, and tihs allows us to move the
title formatting code to the RSS serializers.

app/serializers/oembed_serializer.rb

@@ -4,7 +4,7 @@ class OEmbedSerializer < ActiveModel::Serializer
   include RoutingHelper
   include ActionView::Helpers::TagHelper
 
-  attributes :type, :version, :title, :author_name,
+  attributes :type, :version, :author_name,
              :author_url, :provider_name, :provider_url,
              :cache_age, :html, :width, :height