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)

Using pseudo-elements means there won't be a png fallback. 
I'm down with that, as long as the icon isn't integral or there's a fallback, but it's something we should discuss at the next cs metting