Prevent anon/banned/member users from voting (fix #2719).

There was a regression in 6d6d00b; `before_filter :voter_only` was a
no-op in the post vote controller because it merely returned false,
which does not halt the request. The fix is to arrange for a voter_only
method to be defined that properly redirects to the access denied page.

app/models/user.rb

@@ -16,6 +16,16 @@ class User < ActiveRecord::Base
     ADMIN = 50
   end
 
+  # Used for `before_filter :<role>_only`. Must have a corresponding `is_<role>?` method.
+  Roles = Levels.constants.map(&:downcase) + [
+    :anonymous,
+    :banned,
+    :approver,
+    :voter,
+    :super_voter,
+    :verified,
+  ]
+
   BOOLEAN_ATTRIBUTES = %w(
     is_banned
     has_mail
@@ -384,6 +394,10 @@ class User < ActiveRecord::Base
       true
     end
 
+    def is_blocked?
+      is_banned?
+    end
+
     def is_builder?
       level >= Levels::BUILDER
     end