views: factor out FontAwesome icons.

Factor out FontAwesome icons into a set of helpers. This is so that it's
easier to keep track of which icons we're using and easier to change
icons globally.

app/helpers/forum_topics_helper.rb

@@ -20,4 +20,14 @@ module ForumTopicsHelper
       :rejected
     end
   end
+
+  def forum_post_vote_icon(vote)
+    if vote.score == 1
+      upvote_icon
+    elsif vote.score == -1
+      downvote_icon
+    else
+      meh_icon
+    end
+  end
 end