Add primary group classes (PR #12807)

Add primary group CSS class target in group post list, and in usercards.

GitHub

Do we need to ensure the name is CSS-class-friendly?

So I thought about that, but it appears like this is the same format as what we’re doing for printing out the primary group to the post as well: discourse/post.js at master · discourse/discourse · GitHub

I err’d on the side of consistency, and group names can’t have spaces in them anyway.