forum secondary post action labels

@gwprice @talbs

still needs tests, but figured i'd share -
wonder if y'all think this is a fine way to factor/reuse styles (see .forum-post-label-* stuff and the new mixin)

making some layout inferences based on the post action cleanup wiki.  did not extend these changes into thread responses / comments, just the main thread header.
Change the name of the format parameter from `author_display` to just `author` to avoid translator confusion