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.

spec/lib/rss/serializer_spec.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe RSS::Serializer do
+  describe '#status_title' do
+    let(:text)      { 'This is a toot' }
+    let(:spoiler)   { '' }
+    let(:sensitive) { false }
+    let(:reblog)    { nil }
+    let(:account)   { Fabricate(:account) }
+    let(:status)    { Fabricate(:status, account: account, text: text, spoiler_text: spoiler, sensitive: sensitive, reblog: reblog) }
+
+    subject { RSS::Serializer.new.send(:status_title, status) }
+
+    context 'if destroyed?' do
+      it 'returns "#{account.acct} deleted status"' do
+        status.destroy!
+        expect(subject).to eq "#{account.acct} deleted status"
+      end
+    end
+
+    context 'on a toot with long text' do
+      let(:text) { "This toot's text is longer than the allowed number of characters" }
+
+      it 'truncates toot text appropriately' do
+        expect(subject).to eq "#{account.acct}: “This toot's text is longer tha…”"
+      end
+    end
+
+    context 'on a toot with long text with a newline' do
+      let(:text) { "This toot's text is longer\nthan the allowed number of characters" }
+
+      it 'truncates toot text appropriately' do
+        expect(subject).to eq "#{account.acct}: “This toot's text is longer…”"
+      end
+    end
+
+    context 'on a toot with a content warning' do
+      let(:spoiler) { 'long toot' }
+
+      it 'displays spoiler text instead of toot content' do
+        expect(subject).to eq "#{account.acct}: CW “long toot”"
+      end
+    end
+
+    context 'on a toot with sensitive media' do
+      let(:sensitive) { true }
+
+      it 'displays that the media is sensitive' do
+        expect(subject).to eq "#{account.acct}: “This is a toot” (sensitive)"
+      end
+    end
+
+    context 'on a reblog' do
+      let(:reblog) { Fabricate(:status, text: 'This is a toot') }
+
+      it 'display that the toot is a reblog' do
+        expect(subject).to eq "#{account.acct} boosted #{reblog.account.acct}: “This is a toot”"
+      end
+    end
+  end
+end