When upgrading to Django 3.2, we converted all of our template tags and

filters to use mark_safe() instead of setting is_safe = True, with

the exception of showextrawhitespace.
This was turning safe HTML into an unsafe string, and the result was

that any comments shown in comments in a review or in e-mails was

turning into big blobs of escaped HTML.
This change fixes it to use mark_safe(), and adds some unit tests to

ensure this never regresses.

Viewed comments in a diff that exhibited this problem. Saw that the

content was correct in all chunks of the diffs I expanded.
Unit tests pass.