Add sizes attribute to favicon

>The sizes attribute gives the sizes of icons for visual media. Its value, if present, is merely advisory. User agents may use the value to decide which icon(s) to use if multiple icons are available.
_[W3C] (http://www.w3.org/TR/html5/links.html#rel-icon)_

The modern browsers respect the sizes attribute.

I add sizes with values to the default theme favicon, so designers can see how it works.

*Replace PR fo #2426*