app controller: standardize access denied error handling.

Refactor to use `render_error_page` to handle User::PrivilegeError
exceptions. This way these exceptions are logged to New Relic.

Changes:

* Anonymous users aren't automatically redirected to the login page.
  Instead they're taken to the access denied page, which links to the
  login/signup pages.

* JSON/XML error responses return `message` instead of `reason`.

test/functional/post_votes_controller_test.rb

@@ -17,7 +17,8 @@ class PostVotesControllerTest < ActionDispatch::IntegrationTest
       end
 
       should "not allow banned users to vote" do
-        @banned = create(:banned_user)
+        @banned = create(:user)
+        @ban = create(:ban, user: @banned)
         post_auth post_votes_path(post_id: @post.id), @banned, params: {:score => "up", :format => "js"}
         assert_response 403
         assert_equal(0, @post.reload.score)