Change RSS feeds (#18356)

* Change RSS feeds

- Use date and time for titles instead of ellipsized text
- Use full content in body, even when there is a content warning
- Use media extensions

* Change feed icons and add width and height attributes to custom emojis

* Fix custom emoji animate on hover breaking

* Fix tests

app/lib/rss/builder.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+class RSS::Builder
+  attr_reader :dsl
+
+  def self.build
+    new.tap do |builder|
+      yield builder.dsl
+    end.to_xml
+  end
+
+  def initialize
+    @dsl = RSS::Channel.new
+  end
+
+  def to_xml
+    ('<?xml version="1.0" encoding="UTF-8"?>'.dup << Ox.dump(wrap_in_document, effort: :tolerant)).force_encoding('UTF-8')
+  end
+
+  private
+
+  def wrap_in_document
+    Ox::Document.new(version: '1.0').tap do |document|
+      document << Ox::Element.new('rss').tap do |rss|
+        rss['version']        = '2.0'
+        rss['xmlns:webfeeds'] = 'http://webfeeds.org/rss/1.0'
+        rss['xmlns:media']    = 'http://search.yahoo.com/mrss/'
+
+        rss << @dsl.to_element
+      end
+    end
+  end
+end