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

This reverts commit 20780a1eeed56b321daf18ee6bbfe681a51d1bf4.

* 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)

spec/requests/topics_controller_spec.rb

@@ -2127,7 +2127,7 @@ RSpec.describe TopicsController do
     let(:topic) { post.topic }
 
     after do
-      Discourse.redis.flushall
+      Discourse.redis.flushdb
     end
 
     it 'returns first post of the topic' do