Deboxify .nav-stacked on user-page

As per https://meta.discourse.org/t/user-activity-tabs/16308/13, deboxify the lefthand nav on user page and use less cutting color.

Before:
![img](http://jaetkow.se/i/b003cf817f.png)

After:
![img](http://jaetkow.se/i/e6336796c3.png)