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/lib/rss/serializer.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+class RSS::Serializer
+  private
+
+  def render_statuses(builder, statuses)
+    statuses.each do |status|
+      builder.item do |item|
+        item.title(status_title(status))
+            .link(ActivityPub::TagManager.instance.url_for(status))
+            .pub_date(status.created_at)
+            .description(status.spoiler_text.presence || Formatter.instance.format(status, inline_poll_options: true).to_str)
+
+        status.media_attachments.each do |media|
+          item.enclosure(full_asset_url(media.file.url(:original, false)), media.file.content_type, media.file.size)
+        end
+      end
+    end
+  end
+
+  def status_title(status)
+    return "#{status.account.acct} deleted status" if status.destroyed?
+
+    preview = status.proper.spoiler_text.presence || status.proper.text
+    if preview.length > 30 || preview[0, 30].include?("\n")
+      preview = preview[0, 30]
+      preview = preview[0, preview.index("\n").presence || 30] + '…'
+    end
+
+    preview = "#{status.proper.spoiler_text.present? ? 'CW ' : ''}“#{preview}”#{status.proper.sensitive? ? ' (sensitive)' : ''}"
+
+    if status.reblog?
+      "#{status.account.acct} boosted #{status.reblog.account.acct}: #{preview}"
+    else
+      "#{status.account.acct}: #{preview}"
+    end
+  end
+end