discussion style changes

Changes
------------
- inline timestamps
- add permalink icon
- hide actions when discussion is closed
- change relative date format

![image](https://cloud.githubusercontent.com/assets/960919/4560825/ff9af7e6-4ef7-11e4-8ac4-abb8c9987e92.png)

We are using two types of icons across the page. In new slices we seem to be using pseudo-elements for icons. It will be nice to start moving to that everywhere. This <i> element for it is so 2004.

You have a nice unique class `.d-comment__timestamp` so it should be easy to extend/include icon as after pseudo-element.