Added link underline and hover to yellow

https://github.com/webcompat/webcompat.com/issues/421

This is a bit strange - maybe @magsout can fix it.  The new style for links is: 
-underline and hover to yellow
-if background is already yellow, hover to white

I added a class .no-underline which I used on the Footer and on the Browse Issues statuses to remove the underline.