go.tools/dashboard/app: only accept commits when given master key
go.tools/dashboard/app: only accept commits when given master key

R=dvyukov, rsc
CC=golang-dev
https://golang.org/cl/37790044

evandbrown/build,golang/build,proppy/build,proppy/build,golang/build,golang/build,evandbrown/build,proppy/build,golang/build,proppy/build,evandbrown/build