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.
There was a bug here: the current code would open up an editor with the current user prepopulated in the recipients list.

By passing in the user, we can allow sending privates messages to other people while on the topics page (ie by clicking on somebody's avatar and having their card pop up.)