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)

lib/discourse_plugin_registry.rb

@@ -75,6 +75,8 @@ class DiscoursePluginRegistry
 
   define_filtered_register :editable_group_custom_fields
 
+  define_filtered_register :topic_thumbnail_sizes
+
   def self.register_auth_provider(auth_provider)
     self.auth_providers << auth_provider
   end