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/item.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+class RSS::Item < RSS::Element
+  def initialize
+    super()
+
+    @root = create_element('item')
+  end
+
+  def title(str)
+    append_element('title', str)
+  end
+
+  def link(str)
+    append_element('guid', str) do |guid|
+      guid['isPermaLink'] = 'true'
+    end
+
+    append_element('link', str)
+  end
+
+  def pub_date(date)
+    append_element('pubDate', date.to_formatted_s(:rfc822))
+  end
+
+  def description(str)
+    append_element('description', str)
+  end
+
+  def category(str)
+    append_element('category', str)
+  end
+
+  def enclosure(url, type, size)
+    append_element('enclosure') do |enclosure|
+      enclosure['url']    = url
+      enclosure['length'] = size
+      enclosure['type']   = type
+    end
+  end
+
+  def media_content(url, type, size, &block)
+    @root << RSS::MediaContent.with(url, type, size, &block)
+  end
+end