Dashboard icon styling fix

When viewing a fresh install of GitLab on a mobile device, the styling for the dashboard item icon is broken in such a way that if the description div is more than 100px high, its contents wrap beneath the icon as shown below.

![Screen Shot 2015-01-05 at 7.11.37 PM](https://gitlab.com/uploads/cloudwindhk/gitlab-ce/9ac2feca33/Screen_Shot_2015-01-05_at_7.11.37_PM.png)

I modified the css for the icon and added a new class to the description div and now it behaves as expected as shown below.

![Screen Shot 2015-01-05 at 6.56.33 PM](https://gitlab.com/uploads/cloudwindhk/gitlab-ce/25cf3212d2/Screen_Shot_2015-01-05_at_6.56.33_PM.png)

As this is a purely html/css change, I don't believe it will break any tests. The only occurrence of .dashboard-intro-icon is in ./app/views/dashboard/_zero_authorized_projects.html.haml

See merge request !278