FIX: when topic is closed, you can still invite someone to reply

Meta: [When topic is closed, you can still invite someone to reply](http://meta.discourse.org/t/when-topic-is-closed-you-can-still-invite-someone-to-reply/6292)

This disables the `invite` button when the topic is **archived** or **closed**:

![image](https://f.cloud.github.com/assets/362783/451860/953812ca-b2be-11e2-998c-32c9a6de179e.png)