On question pages, don't show answer sorting buttons until there are answers

[cellio]

meta:290806

The answer sorting buttons (Score, Active, Age) serve no function until there are two or more answers (that the current user can see -- this could include deleted answers). Let's not show the sorting buttons until there's something to sort.

I believe that every action that could lead to a new answer showing up involves a page refresh, so we aren't risking an answer "winking in" while the buttons are gone. Some sites work like that, but as far as I can tell, qpixel doesn't -- you won't see new (or undeleted) answers until you refresh the page, at which point we'd recheck whether to show the buttons.