shared.css: Start vectorizing common icons

This is in response to bug 35345.

I am working under the MediaWiki Google Code-In project,
and this is my first commit. I have some experience
with CSS and vector images, so it seems like a good
place to start. This is for the following task:
https://google-melange.appspot.com/gci/task/view/google/gci2013/5795046364282880

Only the Atom feed icon has been vectorized so far,
but I plan on adding more subsequently. Preliminary
testing shows that this approach works fine on Firefox.
Changed so that older browsers use the old PNG image,
while newer ones that support gradients use the SVG.

Bug: 35345
Change-Id: I842690df6934f4f14807dfc8d20ec2c0b0007df6
Merge "shared.css: Start vectorizing common icons"