TOC (Table of Contents) tags should not be rendered in markdown preview or RTE for comments

Table of contents tags ([toc] or [[_toc_]]) are not supported in comments, as talked about in #335382 (comment 653256119).

However there is a problem where they will render for a comment when previewing in the plain text editor or in the rich text editor.

It also seems to be rendered in email comments. This should be limited as well.