app/build: make the favicon smaller
app/build: make the favicon smaller

The favicon is hotlinked from golang.org/favicon.ico, which is now
larger.

Resize it to 16x16.

Fixes golang/go#17022.

Change-Id: Id74eb3bd8c855a042d033664359f04089d7857ff
Reviewed-on: https://go-review.googlesource.com/28770
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build