Fix Post-rendering bug

Ember.Handlebars.normalizePath was being called with an object as
argument which caused it to error out.
also wrapped the return in a SafeString, so the html will not get
escaped again.
This fixes the [error with post-rendering][1] after migrating a forum to v0.9.81

This is not very pretty... I'm happy for any suggestions.

[1]: https://meta.discourse.org/t/error-after-update-to-0-9-8-1/11903