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/channel.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+class RSS::Channel < RSS::Element
+  def initialize
+    super()
+
+    @root = create_element('channel')
+  end
+
+  def title(str)
+    append_element('title', str)
+  end
+
+  def link(str)
+    append_element('link', str)
+  end
+
+  def last_build_date(date)
+    append_element('lastBuildDate', date.to_formatted_s(:rfc822))
+  end
+
+  def image(url, title, link)
+    append_element('image') do |image|
+      image << create_element('url', url)
+      image << create_element('title', title)
+      image << create_element('link', link)
+    end
+  end
+
+  def description(str)
+    append_element('description', str)
+  end
+
+  def generator(str)
+    append_element('generator', str)
+  end
+
+  def icon(str)
+    append_element('webfeeds:icon', str)
+  end
+
+  def logo(str)
+    append_element('webfeeds:logo', str)
+  end
+
+  def item(&block)
+    @root << RSS::Item.with(&block)
+  end
+end