DEV: use HTML5 version of loofah (#21522)

https://meta.discourse.org/t/markdown-preview-and-result-differ/263878

The result of this markdown had different results in the composer preview and the post. This is solved by updating Loofah to the latest version and using html5 fragments like our user had reported. While the change was only needed in cooked_post_processor.rb for this fix, other areas also had to be updated due to various side effects.

app/models/post_hotlinked_media.rb

@@ -10,10 +10,10 @@ class PostHotlinkedMedia < ActiveRecord::Base
          upload_create_failed: "upload_create_failed",
        }
 
-  def self.normalize_src(src)
+  def self.normalize_src(src, reset_scheme: true)
     uri = Addressable::URI.heuristic_parse(src)
     uri.normalize!
-    uri.scheme = nil
+    uri.scheme = nil if reset_scheme
     uri.to_s
   rescue URI::Error, Addressable::URI::InvalidURIError
     src