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/element.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class RSS::Element
+  def self.with(*args, &block)
+    new(*args).tap(&block).to_element
+  end
+
+  def create_element(name, content = nil)
+    Ox::Element.new(name).tap do |element|
+      yield element if block_given?
+      element << content if content.present?
+    end
+  end
+
+  def append_element(name, content = nil)
+    @root << create_element(name, content).tap do |element|
+      yield element if block_given?
+    end
+  end
+
+  def to_element
+    @root
+  end
+end