New Private Message button for user topics

This adds a new private message button to the user topics list, which opens up an empty send private message form.

To wit:

Before
![screenshot from 2014-11-30 15 13 29](https://cloud.githubusercontent.com/assets/750477/5236555/9ce53654-78a3-11e4-8b3d-7429d826b6aa.png)

After
![screenshot from 2014-11-30 15 14 18](https://cloud.githubusercontent.com/assets/750477/5236556/9ce5e130-78a3-11e4-92d4-07c489300951.png)

Could use some guidance around where the front-end tests for this stuff should go; I didn't find a whole lot of coverage under test/controllers, for example.
Am I right in thinking this user will always be the current user? Or is there a case where someone else (an admin?) is viewing another person's topic list?

We only use this to determine whether the button shows up, ie. whether the user has permission to send private messages.