Even out padding on "plus" button in breadcrumb menu

Changed the padding on the ```+``` button to be ```6px``` on all sides.

![Screen_Shot_2016-12-16_at_8_06_07_PM](/uploads/cc85f8ff36c7d2dd75546a67445af19a/Screen_Shot_2016-12-16_at_8_06_07_PM.png)

Closes #25740

See merge request !8154