fix tofu for FontAwesome icons in Backgrids

This was a mistake in commit 8d5ae86c (upgrade to FontAwesome 4.2.1).
The .fa-icon() class is needed so that we have the right font-family and
other properties set. I never noticed the problem because I had
FontAwesome installed systemwide and so my browser was finding the
glyphs there.

Change-Id: I02ab234c8c6db6a0ca3bdc3605dd91b8f9f313da