Adds class to html for subreddit RES style on/off
Adds a class to the dom element "html" when on a subreddit based on the inclusion of the subreddit's custom CSS. If it's on, "html" will have the "yes-subr-style" class, and if it's off, "html" will have the "no-subr-style" class.
Provides no directly visible impact for end users and is only useful for CSS/JS alterations for reddit.
