Use FA GitLab Icon for import project button

Before:

![Screen_Shot_2016-08-08_at_09.43.47](/uploads/e419dc0d229ff848294cf277ffbb02f9/Screen_Shot_2016-08-08_at_09.43.47.png)

After:

![Screen_Shot_2016-08-08_at_09.43.09](/uploads/3a77c5e98385bbc1c7139300eeca5866/Screen_Shot_2016-08-08_at_09.43.09.png)

See merge request !5702