Make even more CSS external

GWT's class obfuscation makes it impossible for users to write
their own CSS that won't break on upgrade. This further helps preserve
class names so uses can continue to rely on the names and use
GerritSite.css to it full ability.
Replace favicon with aokp icon
add gravatar to core plugins
Update GreenCheck and RedX icons

These icons work better for a dark background and don't leave pixels
around the edges.