Merge branch 'fix/style-empty-groups' into 'master'

Wrap "No groups found" message with a .nothing-here-block

The "No groups found" message's style is inconsistent with the other empty messages.

## Screenshots

### Empty groups
![Screen_Shot_2016-05-13_at_13.40.34](/uploads/1447c5f60cde7d50d6f8f07c41704e5c/Screen_Shot_2016-05-13_at_13.40.34.png)

### Empty projects
![Screen_Shot_2016-05-13_at_13.40.30](/uploads/c21ff093f3d6bdbafecf7b075fa33d3c/Screen_Shot_2016-05-13_at_13.40.30.png)

See merge request !4137

app/views/shared/groups/_list.html.haml

@@ -3,4 +3,4 @@
     - groups.each_with_index do |group, i|
       = render "shared/groups/group", group: group
 - else
-  %h3 No groups found
+  .nothing-here-block No groups found