Revert "Revert "Merge branch 'master' of https://github.com/discourse/discourse""

This reverts commit 20780a1eee.

* SECURITY: re-adds accidentally reverted commit:
  03d26cd6: ensure embed_url contains valid http(s) uri
* when the merge commit e62a85cf was reverted, git chose the 2660c2e2 parent to land on
  instead of the 03d26cd6 parent (which contains security fixes)

app/models/topic_embed.rb

@@ -109,6 +109,8 @@ class TopicEmbed < ActiveRecord::Base
 
     url = UrlHelper.escape_uri(url)
     original_uri = URI.parse(url)
+    raise URI::InvalidURIError unless original_uri.is_a?(URI::HTTP)
+
     opts = {
       tags: %w[div p code pre h1 h2 h3 b em i strong a img ul li ol blockquote],
       attributes: %w[href src class],