Use rails helper image-url for reference images in css

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Added part about the help to markdown
Fix group_icon spec

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge pull request #6447 from jvanbaarsen/patch-2

Added part about the help to markdown
 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