Implement compact side nav

## What does this MR do?
Implements new compact nav design   
Note- new navbar icons are in a separate MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4268)

## What are the relevant issue numbers?
Closes #18013 

## Screenshots (if relevant)
![Screen_Shot_2016-06-02_at_3.20.37_PM](/uploads/d31bd17ae47743f1a20d7429b4396e84/Screen_Shot_2016-06-02_at_3.20.37_PM.png)   
![Screen_Shot_2016-06-02_at_3.20.43_PM](/uploads/d03a080a57707b6a2161127f3904fcd5/Screen_Shot_2016-06-02_at_3.20.43_PM.png)

cc @dzaporozhets 

See merge request !4447