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/models/forum_post_vote.rb

@@ -36,16 +36,6 @@ class ForumPostVote < ApplicationRecord
     score == 0
   end
 
-  def fa_class
-    if score == 1
-      return "fa-thumbs-up"
-    elsif score == -1
-      return "fa-thumbs-down"
-    else
-      return "fa-meh"
-    end
-  end
-
   def vote_type
     if score == 1
       return "up"