Don't render 'Reply' button when user can't post

This removes the 'Reply' button from the topic footer when a user cannot post a reply. See [conversation on meta](https://meta.discourse.org/t/closed-topics-should-change-the-hovertext-of-reply-button/10970/5).

![image](https://f.cloud.github.com/assets/3401659/2174162/28799ba0-95ac-11e3-95c9-cd6bdbdec076.png)