dashboard/builders: add github handles

Adds an initial set of github handles for builder owners and displays a link in
coordinator if set.

Updates golang/go#19848

Change-Id: I85aa98a48b8a094482b22021b32b465af5313bbe
Reviewed-on: https://go-review.googlesource.com/40654
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed