[SE-4690] Discussion xss fix

[pkulkark]

(cherry picked from commit 82939c3)

This backports edx/edx-platform#28371 to Esme's custom Koa branch.

Testing instructions

1. Check that the diff is matching the original PR.

[gabor-boros]

👍 🎉 @pkulkark

Reminder: we will need to start new 3 app servers using this branch from now on.

• I tested what's in the test instructions
• I read through the code
• I checked for accessibility issues
• Includes documentation
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository.