[release-branch.go1.7] website: recreate 16px and 32px favicon

Recreated original favicon with svg. Note, the rasterizations are hand
tweaked for crispness and straight export will not give the same results.

Fixes #6938

Change-Id: I9bf7b59028711361c29365b145932d90af419b69
Reviewed-on: https://go-review.googlesource.com/26850Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-on: https://go-review.googlesource.com/28639Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>