Better empty state for Groups view.

Implemented @hazelyang's [awesome design](https://gitlab.com/gitlab-org/gitlab-ce/uploads/9141470ed121f137a4074ae4e12c449c/groups-empty.png) for empty group views.

Fixes #20829 


### Design
![https://gitlab.com/gitlab-org/gitlab-ce/uploads/9141470ed121f137a4074ae4e12c449c/groups-empty.png](https://gitlab.com/gitlab-org/gitlab-ce/uploads/9141470ed121f137a4074ae4e12c449c/groups-empty.png)


### Screenshots
![Screen_Shot_2016-09-28_at_13.43.35](/uploads/e2ab668a412596205deb67c97af78a1d/Screen_Shot_2016-09-28_at_13.43.35.png)

![Screen_Shot_2016-09-28_at_13.44.54](/uploads/c4301166f353f0ee198aecdc46cdfe66/Screen_Shot_2016-09-28_at_13.44.54.png)

![Screen_Shot_2016-09-28_at_13.46.10](/uploads/92024e866871969ec7a28db4b8e87220/Screen_Shot_2016-09-28_at_13.46.10.png)

![Screen_Shot_2016-09-28_at_13.46.20](/uploads/f1b515c7aa09d06e80a419d98ff76fd6/Screen_Shot_2016-09-28_at_13.46.20.png)

![Screen_Shot_2016-09-28_at_13.46.34](/uploads/76cdd5f0e9d3c59d017df26172f7dd4d/Screen_Shot_2016-09-28_at_13.46.34.png)

See merge request !6566