Use rails helper image-url for reference images in css

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Fix group_icon spec

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
 Merge branch 'assets-css-helpers' into 'master'

Use rails helper image-url for reference images in css

Fixes #1001
Merge branch 'master' of github.com:gitlabhq/gitlabhq