Bug 7500 [FOLLOW-UP, revised][Social networks] Use CSS Sprites for faster page loading

Bug 7500 [FOLLOW-UP, revised][Social networks] Use CSS Sprites for faster page loading

This patch adds a new sprite image for social network links.
The page markup has been modified to accommodate the image-
replacement technique used to display the images.

When testing be sure to clear your cache to load the new CSS.

Revision: Rebased, and unused social network images removed.