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.
`Discourse.User.current` is a singleton available everywhere (there's also the `HasCurrentUser` mixin if you want) so there's no need to pass it to the `user_topics_list` controller ;)