reduce favicon from 362K -> 9K

The favicon as I submitted it included image sizes as high as 256px, thinking in terms of uses of the file for high-resolution scenarios.

However, now that I think about it, favicons are supposed to have small file sizes, so as to be quick to download. github.com/favicon.ico has a 16px resolution image and a 32px resolution image and that's it. In scenarios where a larger image is needed (like Apple touch icons), a separate image is normally used (and a 256px PNG is only 10kb, unlike a bitmapped .ico).

This commit reduces the size of favicon.ico. Rather than trying to pull from the ICO, larger size applications should derive from either the SVG directly or PNG renders of the SVG.