"New Topic" button doesn't work on single forum topic page

[cnorris23]

If you're on a single topic page in the group forums, the "New Topic" button is shown. However, it doesn't do anything. I don't know if this is an oversight, or intentional. I'm guessing it's just an oversight, and in this case, all that's required is a check to see if we're on a topic page. I have included a patch for this.

[johnjamesjacoby]

(In [3108]) Fixes #2478 props cnorris23