Zen Mode for issues/MR/notes

##### What does this PR do?

This PR implements a so called **Zen** mode. Users have the ability to zwitch into a fullscreen mode, to have more area for writing a

* issue
* merge request
* note

An image says more than 1000 words:
![screenshot 2014-09-11 12 24 20](https://cloud.githubusercontent.com/assets/278301/4236731/4c77bf98-39c9-11e4-940c-850a4c55aa69.png)



Would it be possible to nest this CSS values the scss way?