Use warning color merge conflicts icon in MR status widget.

Fixes #18174 

### Before
![Screen_Shot_2016-06-06_at_18.00.56](/uploads/4a74b1710478daee5ee95b7c6b6a835f/Screen_Shot_2016-06-06_at_18.00.56.png)

### After
![Screen_Shot_2016-06-06_at_17.58.04](/uploads/28faefaebef30e4256390029c87b28bb/Screen_Shot_2016-06-06_at_17.58.04.png)

See merge request !4487