pundit: convert post votes to pundit.

Side effects:

* The data-current-user-is-voter <body> attribute has been removed.
* {{upvote:self}} no longer works. {{upvote:<name>}} should be used instead.

app/policies/post_vote_policy.rb

@@ -0,0 +1,9 @@
+class PostVotePolicy < ApplicationPolicy
+  def create?
+    unbanned? && user.is_gold?
+  end
+
+  def destroy?
+    unbanned? && user.is_gold?
+  end
+end