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)

app/controllers/about_controller.rb

@@ -16,7 +16,7 @@ class AboutController < ApplicationController
         render :index
       end
       format.json do
-        render_serialized(@about, AboutSerializer)
+        render_json_dump(AboutSerializer.new(@about, scope: guardian))
       end
     end
   end