Restricted HTML comments and preview feature

Developed for a client. The preview feature is agnostic of any serverside parsing, so can also be used if we decide to implement shortcodes or markdown processing in comments.
I've checked how the preview renders in the default "simple" theme, and its not really any better or worse than the default comment formatting :) I've ensured the preview button is styled as a secondary action there, which is the most important bit (https://github.com/silverstripe-themes/silverstripe-simple/commit/c991a8eb3282059355cee0fe6e9224422263fa4a)